See also the Policy controls section above.

Those options that undergo string expansion before use are marked with †.

This option causes Exim to send 8BITMIME in its response to an SMTP EHLO command, and to accept the BODY= parameter on MAIL commands. However, though Exim is 8-bit clean, it is not a protocol converter, and it takes no steps to do anything special with messages received by this route. Consequently, this option is turned off by default.

This option defines the ACL that is run when a non-SMTP message is on the point of being accepted. See chapter 39 for further details.

This option defines the ACL that is run when an SMTP AUTH command is received. See chapter 39 for further details.

This option defines the ACL that is run when an SMTP connection is received. See chapter 39 for further details.

This option defines the ACL that is run after an SMTP DATA command has been processed and the message itself has been received, but before the final acknowledgement is sent. See chapter 39 for further details.

This option defines the ACL that is run when an SMTP ETRN command is received. See chapter 39 for further details.

This option defines the ACL that is run when an SMTP EXPN command is received. See chapter 39 for further details.

This option defines the ACL that is run when an SMTP EHLO or HELO command is received. See chapter 39 for further details.

This option defines the ACL that is run when an SMTP MAIL command is received. See chapter 39 for further details.

This option defines the ACL that is run when there is an AUTH parameter on a MAIL command. See chapter 39 for details of ACLs, and chapter 33 for details of authentication.

This option is available when Exim is built with the content-scanning extension. It defines the ACL that is run for each MIME part in a message. See section 40.4 for details.

This option defines the ACL that is run when an SMTP DATA command is received, before the message itself is received. See chapter 39 for further details.

This option defines the ACL that is run when an SMTP QUIT command is received. See chapter 39 for further details.

This option defines the ACL that is run when an SMTP RCPT command is received. See chapter 39 for further details.

This option defines the ACL that is run when an SMTP STARTTLS command is received. See chapter 39 for further details.

This option defines the ACL that is run when an SMTP VRFY command is received. See chapter 39 for further details.

If this option is set, the RFC 2822 domain literal format is permitted in email addresses. The option is not set by default, because the domain literal format is not normally required these days, and few people know about it. It has, however, been exploited by mail abusers.

Unfortunately, it seems that some DNS black list maintainers are using this format to report black listing to postmasters. If you want to accept messages addressed to your hosts by IP address, you need to set allow_domain_literals true, and also to add @[] to the list of local domains (defined in the named domain list local_domains in the default configuration). This “magic string” matches the domain literal form of all the local host’s IP addresses.

It appears that more and more DNS zone administrators are breaking the rules and putting domain names that look like IP addresses on the right hand side of MX records. Exim follows the rules and rejects this, giving an error message that explains the mis-configuration. However, some other MTAs support this practice, so to avoid ``Why can't Exim do this? complaints, allow_mx_to_ip exists, in order to enable this heinous activity. It is not recommended, except when you have no other choice.

Lots of discussion is going on about internationalized domain names. One camp is strongly in favour of just using UTF-8 characters, and it seems that at least two other MTAs permit this. This option allows Exim users to experiment if they wish.

If it is set true, Exim’s domain parsing function allows valid UTF-8 multicharacters to appear in domain name components, in addition to letters, digits, and hyphens. However, just setting this option is not enough; if you want to look up these domain names in the DNS, you must also adjust the value of dns_check_names_pattern to match the extended form. A suitable setting is:

dns_check_names_pattern = (?i)^(?>(?(1)\.|())[a-z0-9\xc0-\xff]\
  (?>[-a-z0-9\x80-\xff]*[a-z0-9\x80-\xbf])?)+$

Alternatively, you can just disable this feature by setting

dns_check_names_pattern =

That is, set the option to an empty string so that no check is done.

If any server authentication mechanisms are configured, Exim advertises them in response to an EHLO command only if the calling host matches this list. Otherwise, Exim does not advertise AUTH. Exim does not accept AUTH commands from clients to which it has not advertised the availability of AUTH. The advertising of individual authentication mechanisms can be controlled by the use of the server_advertise_condition generic authenticator option on the individual authenticators. See chapter 33 for further details.

Certain mail clients (for example, Netscape) require the user to provide a name and password for authentication if AUTH is advertised, even though it may not be needed (the host may accept messages from hosts on its local LAN without authentication, for example). The auth_advertise_hosts option can be used to make these clients more friendly by excluding them from the set of hosts to which Exim advertises AUTH.

If you want to advertise the availability of AUTH only when the connection is encrypted using TLS, you can make use of the fact that the value of this option is expanded, with a setting like this:

auth_advertise_hosts = ${if eq{$tls_cipher}{}{}{*}}

If $tls_cipher is empty, the session is not encrypted, and the result of the expansion is empty, thus matching no hosts. Otherwise, the result of the expansion is *, which matches all hosts.

Note: This is an old option, which predates timeout_frozen_after and ignore_bounce_errors_after. It is retained for compatibility, but it is not thought to be very useful any more, and its use should probably be avoided.

This option is available if Exim is built with the content-scanning extension. It specifies which anti-virus scanner to use. The default value is:

sophie:/var/run/sophie

If the value of av_scanner starts with dollar character, it is expanded before use. See section 40.1 for further details.

This option supplies the name of a command that is run when Exim is called with the -bi option (see chapter 5). The string value is just the command name, it is not a complete command line. If an argument is required, it must come from the -oA command line option.

This option defines a template file containing paragraphs of text to be used for constructing bounce messages. Details of the file’s contents are given in chapter 45. See also warn_message_file.

When this option is set, its contents are included in the default bounce message immediately after “This message was created automatically by mail delivery software.” It is not used if bounce_message_file is set.

This option controls whether the body of an incoming message is included in a bounce message when bounce_return_message is true. If it is not set, only the message header is included.

If this option is set false, the original message is not included in bounce messages generated by Exim. See also bounce_return_size_limit.

This option sets a limit in bytes on the size of messages that are returned to senders as part of bounce messages when bounce_return_message is true. The limit should be less than the value of the global message_size_limit and of any message_size_limit settings on transports, to allow for the bounce text that Exim generates. If this option is set to zero there is no limit.

When the body of any message that is to be included in a bounce message is greater than the limit, it is truncated, and a comment pointing this out is added at the top. The actual cutoff may be greater than the value given, owing to the use of buffering for transferring the message in chunks (typically 8K in size). The idea is to save bandwidth on those undeliverable 15-megabyte messages.

This option provides an authenticated sender address that is sent with any bounce messages generated by Exim that are sent over an authenticated SMTP connection. A typical setting might be:

bounce_sender_authentication = mailer-daemon@my.domain.example

which would cause bounce messages to be sent using the SMTP command:

MAIL FROM:<> AUTH=mailer-daemon@my.domain.example

The value of bounce_sender_authentication must always be a complete email address.

This option specifies the expiry time for negative callout cache data for a domain. See section 39.32 for details of callout verification, and section 39.34 for details of the caching.

This option specifies the expiry time for positive callout cache data for a domain. See section 39.32 for details of callout verification, and section 39.34 for details of the caching.

This option specifies the expiry time for negative callout cache data for an address. See section 39.32 for details of callout verification, and section 39.34 for details of the caching.

This option specifies the expiry time for positive callout cache data for an address. See section 39.32 for details of callout verification, and section 39.34 for details of the caching.

This option defines the “random” local part that can be used as part of callout verification. The default value is

$primary_host_name-$tod_epoch-testing

See section 39.33 for details of how this value is used.

See check_spool_space below.

See check_spool_space below.

RFC 2047 defines a way of encoding non-ASCII characters in headers using a system of “encoded words”. The RFC specifies a maximum length for an encoded word; strings to be encoded that exceed this length are supposed to use multiple encoded words. By default, Exim does not recognize encoded words that exceed the maximum length. However, it seems that some software, in violation of the RFC, generates overlong encoded words. If check_rfc2047_length is set false, Exim recognizes encoded words of any length.

See check_spool_space below.

The four check_… options allow for checking of disk resources before a message is accepted.

When any of these options are set, they apply to all incoming messages. If you want to apply different checks to different kinds of message, you can do so by testing the the variables $log_inodes, $log_space, $spool_inodes, and $spool_space in an ACL with appropriate additional conditions.

check_spool_space and check_spool_inodes check the spool partition if either value is greater than zero, for example:

check_spool_space = 10M
check_spool_inodes = 100

The spool partition is the one that contains the directory defined by SPOOL_DIRECTORY in Local/Makefile. It is used for holding messages in transit.

check_log_space and check_log_inodes check the partition in which log files are written if either is greater than zero. These should be set only if log_file_path and spool_directory refer to different partitions.

If there is less space or fewer inodes than requested, Exim refuses to accept incoming mail. In the case of SMTP input this is done by giving a 452 temporary error response to the MAIL command. If ESMTP is in use and there was a SIZE parameter on the MAIL command, its value is added to the check_spool_space value, and the check is performed even if check_spool_space is zero, unless no_smtp_check_spool_space is set.

The values for check_spool_space and check_log_space are held as a number of kilobytes. If a non-multiple of 1024 is specified, it is rounded up.

For non-SMTP input and for batched SMTP input, the test is done at start-up; on failure a message is written to stderr and Exim exits with a non-zero code, as it obviously cannot send an error message of any kind.

This option specifies one or more default SMTP ports on which the Exim daemon listens. See chapter 13 for details of how it is used. For backward compatibility, daemon_smtp_port (singular) is a synonym.

When a message is delayed, Exim sends a warning message to the sender at intervals specified by this option. The data is a colon-separated list of times after which to send warning messages.

If the value of the option is an empty string or a zero time, no warnings are sent.

Up to 10 times may be given. If a message has been on the queue for longer than the last time, the last interval between the times is used to compute subsequent warning times. For example, with

delay_warning = 4h:8h:24h

the first message is sent after 4 hours, the second after 8 hours, and the third one after 24 hours. After that, messages are sent every 16 hours, because that is the interval between the last two times on the list. If you set just one time, it specifies the repeat interval. For example, with:

delay_warning = 6h

messages are repeated every six hours. To stop warnings after a given time, set a very large time at the end of the list. For example:

delay_warning = 2h:12h:99d

The string is expanded at the time a warning message might be sent. If all the deferred addresses have the same domain, it is set in $domain during the expansion. Otherwise $domain is empty. If the result of the expansion is a forced failure, an empty string, or a string matching any of “0”, “no” or “false” (the comparison being done caselessly) then the warning message is not sent. The default is

delay_warning_condition = \
  ${if match{$h_precedence:}{(?i)bulk|list|junk}{no}{yes}}

which suppresses the sending of warnings about messages that have “bulk”, “list” or “junk” in a Precedence: header.

If this option is set true, Exim drops its root privilege at the start of a delivery process, and runs as the Exim user throughout. This severely restricts the kinds of local delivery that are possible, but is viable in certain types of configuration. There is a discussion about the use of root privilege in chapter 51.

When this option is set, a queue run is abandoned if the system load average becomes greater than the value of the option. The option has no effect on ancient operating systems on which Exim cannot determine the load average. See also queue_only_load and smtp_load_reserve.

Exim’s transports have an option for adding a Delivery-date: header to a message when it is delivered – in exactly the same way as Return-path: is handled. Delivery-date: records the actual time of delivery. Such headers should not be present in incoming messages, and this option causes them to be removed at the time the message is received, to avoid any problems that might occur when a delivered message is subsequently sent on to some other recipient.

DNS lookups give a “try again” response for the DNS errors “non-authoritative host not found” and “SERVERFAIL”. This can cause Exim to keep trying to deliver a message, or to give repeated temporary errors to incoming mail. Sometimes the effect is caused by a badly set up name server and may persist for a long time. If a domain which exhibits this problem matches anything in dns_again_means_nonexist, it is treated as if it did not exist. This option should be used with care. You can make it apply to reverse lookups by a setting such as this:

dns_again_means_nonexist = *.in-addr.arpa

This option applies to all DNS lookups that Exim does. The dnslookup router has some options of its own for controlling what happens when lookups for MX or SRV records give temporary errors. These more specific options are applied after the global option.

When this option is set to a non-empty string, it causes Exim to check domain names for illegal characters before handing them to the DNS resolver, because some resolvers give temporary errors for malformed names. If a domain name contains any illegal characters, a “not found” result is forced, and the resolver is not called. The check is done by matching the domain name against a regular expression, which is the value of this option. The default pattern is

dns_check_names_pattern = \
  (?i)^(?>(?(1)\.|())[^\W_](?>[a-z0-9-]*[^\W_])?)+$

which permits only letters, digits, and hyphens in components, but they may not start or end with a hyphen. If you set allow_utf8_domains, you must modify this pattern, or set the option to an empty string.

When Exim is compiled with IPv6 support, it looks for IPv6 address records (AAAA and, if configured, A6) as well as IPv4 address records when trying to find IP addresses for hosts, unless the host’s domain matches this list.

This is a fudge to help with name servers that give big delays or otherwise do not work for the new IPv6 record types. If Exim is handed an IPv6 address record as a result of an MX lookup, it always recognizes it, and may as a result make an outgoing IPv6 connection. All this option does is to make Exim look only for IPv4-style A records when it needs to find an IP address for a host name. In due course, when the world’s name servers have all been upgraded, there should be no need for this option.

The options dns_retrans and dns_retry can be used to set the retransmission and retry parameters for DNS lookups. Values of zero (the defaults) leave the system default settings unchanged. The first value is the time between retries, and the second is the number of retries. It isn’t totally clear exactly how these settings affect the total time a DNS lookup may take. I haven’t found any documentation about timeouts on DNS lookups; these parameter values are available in the external resolver interface structure, but nowhere does it seem to describe how they are used or what you might want to set in them.

See dns_retrans above.

This is an obsolete option that is now a no-op. It used to affect the way Exim handled CR and LF characters in incoming messages. What happens now is described in section 43.2.

Exim’s transports have an option for adding an Envelope-to: header to a message when it is delivered – in exactly the same way as Return-path: is handled. Envelope-to: records the original recipient address from the messages’s envelope that caused the delivery to happen. Such headers should not be present in incoming messages, and this option causes them to be removed at the time the message is received, to avoid any problems that might occur when a delivered message is subsequently sent on to some other recipient.

Setting this option causes Exim to send bcc copies of bounce messages that it generates to other addresses. Note: this does not apply to bounce messages coming from elsewhere. The value of the option is a colon-separated list of items. Each item consists of a pattern, terminated by white space, followed by a comma-separated list of email addresses. If a pattern contains spaces, it must be enclosed in double quotes.

Each pattern is processed in the same way as a single item in an address list (see section 10.18). When a pattern matches the recipient of the bounce message, the message is copied to the addresses on the list. The items are scanned in order, and once a matching one is found, no further items are examined. For example:

errors_copy = spqr@mydomain   postmaster@mydomain.example :\
              rqps@mydomain   hostmaster@mydomain.example,\
                              postmaster@mydomain.example

The address list is expanded before use. The expansion variables $local_part and $domain are set from the original recipient of the error message, and if there was any wildcard matching in the pattern, the expansion

variables $0, $1, etc. are set in the normal way.

Exim’s bounce and delivery warning messages contain the header line

From: Mail Delivery System <Mailer-Daemon@<qualify-domain>>

where <qualify-domain> is the value of the qualify_domain option. Experience shows that people reply to bounce messages. If the errors_reply_to option is set, a Reply-To: header is added to bounce and warning messages. For example:

errors_reply_to = postmaster@my.domain.example

The value of the option is not expanded. It must specify a valid RFC 2822 address.

This option changes the gid under which Exim runs when it gives up root privilege. The default value is compiled into the binary. The value of this option is used only when exim_user is also set. Unless it consists entirely of digits, the string is looked up using getgrnam(), and failure causes a configuration error. See chapter 51 for a discussion of security issues.

This option specifies the path name of the Exim binary, which is used when Exim needs to re-exec itself. The default is set up to point to the file exim in the directory configured at compile time by the BIN_DIRECTORY setting. It is necessary to change exim_path if, exceptionally, Exim is run from some other place. Warning: Do not use a macro to define the value of this option, because you will break those Exim utilities that scan the configuration file to find where the binary is. (They then use the -bP option to extract option settings such as the value of spool_directory.)

This option changes the uid under which Exim runs when it gives up root privilege. The default value is compiled into the binary. Ownership of the run time configuration file and the use of the -C and -D command line options is checked against the values in the binary, not what is set here.

Unless it consists entirely of digits, the string is looked up using getpwnam(), and failure causes a configuration error. If exim_group is not also supplied, the gid is taken from the result of getpwnam() if it is used. See chapter 51 for a discussion of security issues.

This option defines network interfaces that are to be considered local when routing, but which are not used for listening by the daemon. See section 13.7 for details.

According to some Sendmail documentation (Sun, IRIX, HP-UX), if any addresses are present on the command line when the -t option is used to build an envelope from a message’s To:, Cc: and Bcc: headers, the command line addresses are removed from the recipients list. This is also how Smail behaves. However, other Sendmail documentation (the O’Reilly book) states that command line addresses are added to those obtained from the header lines. When extract_addresses_remove_arguments is true (the default), Exim subtracts argument headers. If it is set false, Exim adds rather than removes argument addresses.

On systems running NIS or other schemes in which user and group information is distributed from a remote system, there can be times when getpwnam() and related functions fail, even when given valid data, because things time out. Unfortunately these failures cannot be distinguished from genuine “not found” errors. If finduser_retries is set greater than zero, Exim will try that many extra times to find a user or a group, waiting for one second between retries.

You should not set this option greater than zero if your user information is in a traditional /etc/passwd file, because it will cause Exim needlessly to search the file multiple times for non-existent users, and also cause delay.

On encountering certain errors, or when configured to do so in a system filter, or in an ACL, Exim freezes a message. This means that no further delivery attempts take place until an administrator (or the auto_thaw feature) thaws the message. If freeze_tell is set, Exim generates a warning message whenever it freezes something, unless the message it is freezing is a locally-generated bounce message. (Without this exception there is the possibility of looping.) The warning message is sent to the addresses supplied as the comma-separated value of this option. If several of the message’s addresses cause freezing, only a single message is sent. If the freezing was automatic, the reason(s) for freezing can be found in the message log. If you configure freezing in a filter or ACL, you must arrange for any logging that you require.

Some operating systems, notably HP-UX, use the “gecos” field in the system password file to hold other information in addition to users’ real names. Exim looks up this field for use when it is creating Sender: or From: headers. If either gecos_pattern or gecos_name are unset, the contents of the field are used unchanged, except that, if an ampersand is encountered, it is replaced by the user’s login name with the first character forced to upper case, since this is a convention that is observed on many systems.

When these options are set, gecos_pattern is treated as a regular expression that is to be applied to the field (again with & replaced by the login name), and if it matches, gecos_name is expanded and used as the user’s name.

Numeric variables such as $1, $2, etc. can be used in the expansion to pick up sub-fields that were matched by the pattern. In HP-UX, where the user’s name terminates at the first comma, the following can be used:

gecos_pattern = ([^,]*)
gecos_name = $1

See gecos_name above.

This option sets a default character set for translating from encoded MIME “words” in header lines, when referenced by an $h_xxx expansion item. The default is the value of HEADERS_CHARSET in Local/Makefile. The ultimate default is ISO-8859-1. For more details see the description of header insertions in section 11.5.

This option controls the overall maximum size of a message’s header section. The default is the value of HEADER_MAXSIZE in Local/Makefile; the default for that is 1M. Messages with larger header sections are rejected.

This option limits the length of any individual header line in a message, after all the continuations have been joined together. Messages with individual header lines that are longer than the limit are rejected. The default value of zero means “no limit”.

Exim checks the syntax of HELO and EHLO commands for incoming SMTP mail, and gives an error response for invalid data. Unfortunately, there are some SMTP clients that send syntactic junk. They can be accommodated by setting this option. Note that this is a syntax check only. See helo_verify_hosts if you want to do semantic checking. See also helo_allow_chars for a way of extending the permitted character set.

This option can be set to a string of rogue characters that are permitted in all EHLO and HELO names in addition to the standard letters, digits, hyphens, and dots. If you really must allow underscores, you can set

helo_allow_chars = _

Note that the value is one string, not a list.

If the domain given by a client in a HELO or EHLO command matches this list, a reverse lookup is done in order to establish the host’s true name. The default forces a lookup if the client host gives the server’s name or any of its IP addresses (in brackets), something that broken clients have been seen to do.

However, the EHLO or HELO command is not rejected if any of the checks fail. Processing continues, but the result of the check is remembered, and can be detected later in an ACL by the verify = helo condition.

This option allows mail for particular domains to be held on the queue manually. The option is overridden if a message delivery is forced with the -M, -qf, -Rf or -Sf options, and also while testing or verifying addresses using -bt or -bv. Otherwise, if a domain matches an item in hold_domains, no routing or delivery for that address is done, and it is deferred every time the message is looked at.

This option is intended as a temporary operational measure for delaying the delivery of mail while some problem is being sorted out, or some new configuration tested. If you just want to delay the processing of some domains until a queue run occurs, you should use queue_domains or queue_smtp_domains, not hold_domains.

A setting of hold_domains does not override Exim’s code for removing messages from the queue if they have been there longer than the longest retry time in any retry rule. If you want to hold messages for longer than the normal retry times, insert a dummy retry rule with a long retry time.

Exim does not look up the name of a calling host from its IP address unless it is required to compare against some host list, or the host matches helo_try_verify_hosts or helo_verify_hosts, or the host matches this option (which normally contains IP addresses rather than host names). The default configuration file contains

host_lookup = *

which causes a lookup to happen for all hosts. If the expense of these lookups is felt to be too great, the setting can be changed or removed.

After a successful reverse lookup, Exim does a forward lookup on the name it has obtained, to verify that it yields the IP address that it started with. If this check fails, Exim behaves as if the name lookup failed.

After any kind of failure, the host name (in $sender_host_name) remains unset, and $host_lookup_failed is set to the string “1”. See also dns_again_means_nonexist, helo_lookup_domains, and verify = reverse_host_lookup in ACLs.

This option specifies the order of different lookup methods when Exim is trying to find a host name from an IP address. The default is to do a DNS lookup first, and then to try a local lookup (using gethostbyaddr() or equivalent) if that fails. You can change the order of these lookups, or omit one entirely, if you want.

Warning: the “byaddr” method does not always yield aliases when there are multiple PTR records in the DNS and the IP address is not listed in /etc/hosts. Different operating systems give different results in this case. That is why the default tries a DNS lookup first.

If this option is set, incoming SMTP calls from the hosts listed are rejected as soon as the connection is made. This option is obsolete, and retained only for backward compatibility, because nowadays the ACL specified by acl_smtp_connect can also reject incoming connections immediately.

The ability to give an immediate rejection (either by this option or using an ACL) is provided for use in unusual cases. Many hosts will just try again, sometimes without much delay. Normally, it is better to use an ACL to reject incoming messages at a later stage, such as after RCPT commands. See chapter 39.

This option defines a list of hosts for which connection logging does not happen, even though the smtp_connection log selector is set. For example, you might want not to log SMTP connections from local processes, or from 127.0.0.1, or from your local LAN. This option is consulted in the main loop of the daemon; you should therefore strive to restrict its value to a short inline list of IP addresses and networks. To disable logging SMTP connections from local processes, you must create a host list with an empty item. For example:

hosts_connection_nolog = :

If the smtp_connection log selector is not set, this option has no effect.

If this option is set, any host names that match the domain list are treated as if they were the local host when Exim is scanning host lists obtained from MX records or other sources. Note that the value of this option is a domain list, not a host list, because it is always used to check host names, not IP addresses.

This option also applies when Exim is matching the special items @mx_any, @mx_primary, and @mx_secondary in a domain list (see section 10.8), and when checking the hosts option in the smtp transport for the local host (see the allow_localhost option in that transport). See also local_interfaces, extra_local_interfaces, and chapter 13, which contains a discussion about local network interfaces and recognising the local host.

This option affects the processing of bounce messages that cannot be delivered, that is, those that suffer a permanent delivery failure. (Bounce messages that suffer temporary delivery failures are of course retried in the usual way.)

After a permanent delivery failure, bounce messages are frozen, because there is no sender to whom they can be returned. When a frozen bounce message has been on the queue for more than the given time, it is unfrozen at the next queue run, and a further delivery is attempted. If delivery fails again, the bounce message is discarded. This makes it possible to keep failed bounce messages around for a shorter time than the normal maximum retry time for frozen messages. For example,

ignore_bounce_errors_after = 12h

retries failed bounce message deliveries after 12 hours, discarding any further failures. If the value of this option is set to a zero time period, bounce failures are discarded immediately. Setting a very long time (as in the default value) has the effect of disabling this option. For ways of automatically dealing with other kinds of frozen message, see auto_thaw and timeout_frozen_after.

Some broken SMTP clients insist on sending a UUCP-like “From” line before the headers of a message. By default this is treated as the start of the message’s body, which means that any following headers are not recognized as such. Exim can be made to ignore it by setting ignore_fromline_hosts to match those hosts that insist on sending it. If the sender is actually a local process rather than a remote host, and is using -bs to inject the messages, ignore_fromline_local must be set to achieve this effect.

See ignore_fromline_hosts above.

This option specifies the length of time to keep messages whose spool files have been corrupted in some way. This should, of course, never happen. At the next attempt to deliver such a message, it gets removed. The incident is logged.

This option provides a list of LDAP servers which are tried in turn when an LDAP query does not contain a server. See section 9.14 for details of LDAP queries. This option is available only when Exim has been built with LDAP support.

This option can be used to force Exim to set a specific protocol version for LDAP. If it option is unset, it is shown by the -bP command line option as -1. When this is the case, the default is 3 if LDAP_VERSION3 is defined in the LDAP headers; otherwise it is 2. This option is available only when Exim has been built with LDAP support.

When a message is submitted locally (that is, not over a TCP/IP connection) by an untrusted user, Exim removes any existing Sender: header line, and checks that the From: header line matches the login of the calling user and the domain specified by qualify_domain.

Note: An unqualified address (no domain) in the From: header in a locally submitted message is automatically qualified by Exim, unless the -bnq command line option is used.

You can use local_from_prefix and local_from_suffix to permit affixes on the local part. If the From: header line does not match, Exim adds a Sender: header with an address constructed from the calling user’s login and the default qualify domain.

If local_from_check is set false, the From: header check is disabled, and no Sender: header is ever added. If, in addition, you want to retain Sender: header lines supplied by untrusted users, you must also set local_sender_retain to be true.

These options affect only the header lines in the message. The envelope sender is still forced to be the login id at the qualify domain unless untrusted_set_sender permits the user to supply an envelope sender.

For messages received over TCP/IP, an ACL can specify “submission mode” to request similar header line checking. See section 43.15, which has more details about Sender: processing.

When Exim checks the From: header line of locally submitted messages for matching the login id (see local_from_check above), it can be configured to ignore certain prefixes and suffixes in the local part of the address. This is done by setting local_from_prefix and/or local_from_suffix to appropriate lists, in the same form as the local_part_prefix and local_part_suffix router options (see chapter 15). For example, if

local_from_prefix = *-

is set, a From: line containing

From: anything-user@your.domain.example

will not cause a Sender: header to be added if user@your.domain.example matches the actual sender address that is constructed from the login name and qualify domain.

See local_from_prefix above.

This option controls which network interfaces are used by the daemon for listening; they are also used to identify the local host when routing. Chapter 13 contains a full description of this option and the related options

daemon_smtp_ports, extra_local_interfaces, hosts_treat_as_local, and tls_on_connect_ports.

The default value for local_interfaces is

local_interfaces = 0.0.0.0

when Exim is built without IPv6 support; otherwise it is

local_interfaces = <; ::0 ; 0.0.0.0

This timeout applies to the local_scan() function (see chapter 41). Zero means “no timeout”. If the timeout is exceeded, the incoming message is rejected with a temporary error if it is an SMTP message. For a non-SMTP message, the message is dropped and Exim ends with a non-zero code. The incident is logged on the main and reject logs.

When a message is submitted locally (that is, not over a TCP/IP connection) by an untrusted user, Exim removes any existing Sender: header line. If you do not want this to happen, you must set local_sender_retain, and you must also set local_from_check to be false (Exim will complain if you do not). See also the ACL modifier control = suppress_local_fixups. Section 43.15 has more details about Sender: processing.

Exim’s message ids are normally unique only within the local host. If uniqueness among a set of hosts is required, each host must set a different value for the localhost_number option. The string is expanded immediately after reading the configuration file (so that a number can be computed from the host name, for example) and the result of the expansion must be a number in the range 0–16 (or 0–10 on operating systems with case-insensitive file systems). This is available in subsequent string expansions via the variable $localhost_number. When localhost_number is set, the final two characters of the message id, instead of just being a fractional part of the time, are computed from the time and the local host number as described in section 3.4.

This option sets the path which is used to determine the names of Exim’s log files, or indicates that logging is to be to syslog, or both. It is expanded when Exim is entered, so it can, for example, contain a reference to the host name. If no specific path is set for the log files at compile or run time, they are written in a sub-directory called log in Exim’s spool directory. Chapter 48 contains further details about Exim’s logging, and section 48.1 describes how the contents of log_file_path are used. If this string is fixed at your installation (contains no expansion variables) it is recommended that you do not set this option in the configuration file, but instead supply the path using LOG_FILE_PATH in Local/Makefile so that it is available to Exim for logging errors detected early on – in particular, failure to read the configuration file.

This option can be used to reduce or increase the number of things that Exim writes to its log files. Its argument is made up of names preceded by plus or minus characters. For example:

log_selector = +arguments -retry_defer

A list of possible names and what they control is given in the chapter on logging, in section 48.15.

By default, the timestamps on log lines are in local time without the timezone. This means that if your timezone changes twice a year, the timestamps in log lines are ambiguous for an hour when the clocks go back. One way of avoiding this problem is to set the timezone to UTC. An alternative is to set log_timezone true. This turns on the addition of the timezone offset to timestamps in log lines. Turning on this option can add quite a lot to the size of log files because each line is extended by 6 characters. Note that the $tod_log variable contains the log timestamp without the zone, but there is another variable called $tod_zone that contains just the timezone offset.

This option limits the number of simultaneously open files for single-key lookups that use regular files (that is, lsearch, dbm, and cdb). Exim normally keeps these files open during routing, because often the same file is required several times. If the limit is reached, Exim closes the least recently used file. Note that if you are using the ndbm library, it actually opens two files for each logical DBM database, though it still counts as one for the purposes of lookup_open_max. If you are getting “too many open files” errors with NDBM, you need to reduce the value of lookup_open_max.

Some operating systems are broken in that they truncate long arguments to getpwnam() to eight characters, instead of returning “no such user”. If this option is set greater than zero, any attempt to call getpwnam() with an argument that is longer behaves as if getpwnam() failed.

This option specifies how much of a message’s body is to be included in the $message_body and $message_body_end expansion variables.

If this option is set, the string is expanded and used as the right hand side (domain) of the Message-ID: header that Exim creates if a locally-originated incoming message does not have one. “Locally-originated” means “not received over TCP/IP.” Otherwise, the primary host name is used. Only letters, digits, dot and hyphen are accepted; any other characters are replaced by hyphens. If the expansion is forced to fail, or if the result is an empty string, the option is ignored.

If this variable is set, the string is expanded and used to augment the text of the Message-id: header that Exim creates if a locally-originated incoming message does not have one. The text of this header is required by RFC 2822 to take the form of an address. By default, Exim uses its internal message id as the local part, and the primary host name as the domain. If this option is set, it is expanded, and provided the expansion is not forced to fail, and does not yield an empty string, the result is inserted into the header immediately before the @, separated from the internal message id by a dot. Any characters that are illegal in an address are automatically converted into hyphens. This means that variables such as $tod_log can be used, because the spaces and colons will become hyphens.

If this option is turned off, per-message log files are not created in the msglog spool sub-directory. This reduces the amount of disk I/O required by Exim, by reducing the number of files involved in handling a message from a minimum of four (header spool file, body spool file, delivery journal, and per-message log) to three. The other major I/O activity is Exim’s main log, which is not affected by this option.

This option limits the maximum size of message that Exim will process. The value is expanded for each incoming connection so, for example, it can be made to depend on the IP address of the remote host for messages arriving via TCP/IP. Note: This limit cannot be made to depend on a message’s sender or any other properties of an individual message, because it has to be advertised in the server’s response to EHLO. String expansion failure causes a temporary error. A value of zero means no limit, but its use is not recommended. See also bounce_return_size_limit.

Incoming SMTP messages are failed with a 552 error if the limit is exceeded; locally-generated messages either get a stderr message or a delivery failure message to the sender, depending on the -oe setting. Rejection of an oversized message is logged in both the main and the reject logs. See also the generic transport option message_size_limit, which limits the size of message that an individual transport can process.

This option, which is available only if Exim has been built with the setting

SUPPORT_MOVE_FROZEN_MESSAGES=yes

in Local/Makefile, causes frozen messages and their message logs to be moved from the input and msglog directories on the spool to Finput and Fmsglog, respectively. There is currently no support in Exim or the standard utilities for handling such moved messages, and they do not show up in lists generated by -bp or by the Exim monitor.

Setting this option true causes Exim to run in a very restrictive mode in which it passes messages synchronously to a smart host. Chapter 47 contains a full description of this facility.

This option provides a list of MySQL servers and associated connection data, to be used in conjunction with mysql lookups (see section 9.20). The option is available only if Exim has been built with MySQL support.

When Exim is built, an option called FIXED_NEVER_USERS can be set to a list of users that must not be used for local deliveries. This list is fixed in the binary and cannot be overridden by the configuration file. By default, it contains just the single user name “root”. The never_users runtime option can be used to add more users to the fixed list.

If a message is to be delivered as one of the users on the fixed list or the never_users list, an error occurs, and delivery is deferred. A common example is

never_users = root:daemon:bin

Including root is redundant if it is also on the fixed list, but it does no harm. This option overrides the pipe_as_creator option of the pipe transport driver.

This option provides a list of Oracle servers and associated connection data, to be used in conjunction with oracle lookups (see section 9.20). The option is available only if Exim has been built with Oracle support.

The “percent hack” is the convention whereby a local part containing a percent sign is re-interpreted as a new email address, with the percent replaced by @. This is sometimes called “source routing”, though that term is also applied to RFC 2822 addresses that begin with an @ character. If this option is set, Exim implements the percent facility for those domains listed, but no others. This happens before an incoming SMTP address is tested against an ACL.

Warning: The “percent hack” has often been abused by people who are trying to get round relaying restrictions. For this reason, it is best avoided if at all possible. Unfortunately, a number of less security-conscious MTAs implement it unconditionally. If you are running Exim on a gateway host, and routing mail through to internal MTAs without processing the local parts, it is a good idea to reject recipient addresses with percent characters in their local parts. Exim’s default configuration does this.

This option is available only when Exim is built with an embedded Perl interpreter. See chapter 12 for details of its use.

This option is available only when Exim is built with an embedded Perl interpreter. See chapter 12 for details of its use.

This option provides a list of PostgreSQL servers and associated connection data, to be used in conjunction with pgsql lookups (see section 9.20). The option is available only if Exim has been built with PostgreSQL support.

This option sets the name of the file to which the Exim daemon writes its process id. The string is expanded, so it can contain, for example, references to the host name:

pid_file_path = /var/log/$primary_hostname/exim.pid

If no path is set, the pid is written to the file exim-daemon.pid in Exim’s spool directory. The value set by the option can be overridden by the -oP command line option. A pid file is not written if a “non-standard” daemon is run by means of the -oX option, unless a path is explicitly supplied by -oP.

This option can be used to suppress the advertisement of the SMTP PIPELINING extension to specific hosts. When PIPELINING is not advertised and smtp_enforce_sync is true, an Exim server enforces strict synchronization for each SMTP command and response. When PIPELINING is advertised, Exim assumes that clients will use it; “out of order” commands that are “expected” do not count as protocol errors (see smtp_max_synprot_errors).

If this option is set, message log files are not deleted when messages are completed. Instead, they are moved to a sub-directory of the spool directory called msglog.OLD, where they remain available for statistical or debugging purposes. This is a dangerous option to set on systems with any appreciable volume of mail. Use with care!

This specifies the name of the current host. It is used in the default EHLO or HELO command for outgoing SMTP messages (changeable via the helo_data option in the smtp transport), and as the default for qualify_domain. The value is also used by default in some SMTP response messages from an Exim server. This can be changed dynamically by setting smtp_active_hostname.

If primary_hostname is not set, Exim calls uname() to find the host name. If this fails, Exim panics and dies. If the name returned by uname() contains only one component, Exim passes it to gethostbyname() (or getipnodebyname() when available) in order to obtain the fully qualified version. The variable $primary_hostname contains the host name, whether set explicitly by this option, or defaulted.

By default, Exim considers only those characters whose codes lie in the range 32–126 to be printing characters. In a number of circumstances (for example, when writing log entries) non-printing characters are converted into escape sequences, primarily to avoid messing up the layout. If print_topbitchars is set, code values of 128 and above are also considered to be printing characters.

This option sets the name of the file to which an Exim process writes its “process log” when sent a USR1 signal. This is used by the exiwhat utility script. If this option is unset, the file called exim-process.info in Exim’s spool directory is used. The ability to specify the name explicitly can be useful in environments where two different Exims are running, using different spool directories.

The -M, -R, and -q command-line options require the caller to be an admin user unless prod_requires_admin is set false. See also queue_list_requires_admin.

This option specifies the domain name that is added to any envelope sender addresses that do not have a domain qualification. It also applies to recipient addresses if qualify_recipient is not set.

Unqualified addresses are accepted by default only for locally-generated messages.

Qualification is also applied to addresses in header lines such as From: and To: for locally-generated messages, unless the -bnq command line option is used.

Messages from external sources must always contain fully qualified addresses, unless the sending host matches sender_unqualified_hosts or recipient_unqualified_hosts (as appropriate), in which case incoming addresses are qualified with qualify_domain or qualify_recipient as necessary. Internally, Exim always works with fully qualified envelope addresses. If qualify_domain is not set, it defaults to the primary_hostname value.

This option allows you to specify a different domain for qualifying recipient addresses to the one that is used for senders. See qualify_domain above.

This option lists domains for which immediate delivery is not required. A delivery process is started whenever a message is received, but only those domains that do not match are processed. All other deliveries wait until the next queue run. See also hold_domains and queue_smtp_domains.

The -bp command-line option, which lists the messages that are on the queue, requires the caller to be an admin user unless queue_list_requires_admin is set false. See also prod_requires_admin.

If queue_only is set, a delivery process is not automatically started whenever a message is received. Instead, the message waits on the queue for the next queue run. Even if queue_only is false, incoming messages may not get delivered immediately when certain conditions (such as heavy load) occur.

The -odq command line has the same effect as queue_only. The -odb and -odi command line options override queue_only unless queue_only_override is set false. See also queue_only_file, queue_only_load, and smtp_accept_queue.

This option can be set to a colon-separated list of absolute path names, each one optionally preceded by “smtp”. When Exim is receiving a message, it tests for the existence of each listed path using a call to stat(). For each path that exists, the corresponding queuing option is set. For paths with no prefix, queue_only is set; for paths prefixed by “smtp”, queue_smtp_domains is set to match all domains. So, for example,

queue_only_file = smtp/some/file

causes Exim to behave as if queue_smtp_domains were set to “*” whenever /some/file exists.

If the system load average is higher than this value, incoming messages from all sources are queued, and no automatic deliveries are started. If this happens during local or remote SMTP input, all subsequent messages on the same connection are queued. Deliveries will subsequently be performed by queue runner processes. This option has no effect on ancient operating systems on which Exim cannot determine the load average. See also deliver_queue_load_max and smtp_load_reserve.

When this option is true, the -od’x' command line options override the setting of queue_only or queue_only_file in the configuration file. If queue_only_override is set false, the -od’x' options cannot be used to override; they are accepted, but ignored.

If this option is set, queue runs happen in order of message arrival instead of in an arbitrary order. For this to happen, a complete list of the entire queue must be set up before the deliveries start. When the queue is all held in a single directory (the default),

a single list is created for both the ordered and the non-ordered cases. However, if split_spool_directory is set, a single list is not created when queue_run_in_order is false. In this case, the sub-directories are processed one at a time (in a random order), and this avoids setting up one huge list for the whole queue. Thus, setting queue_run_in_order with split_spool_directory may degrade performance when the queue is large, because of the extra work in setting up the single, large list. In most situations, queue_run_in_order should not be set.

This controls the maximum number of queue runner processes that an Exim daemon can run simultaneously. This does not mean that it starts them all at once, but rather that if the maximum number are still running when the time comes to start another one, it refrains from starting another one. This can happen with very large queues and/or very sluggish deliveries. This option does not, however, interlock with other processes, so additional queue runners can be started by other means, or by killing and restarting the daemon.

When this option is set, a delivery process is started whenever a message is received, routing is performed, and local deliveries take place. However, if any SMTP deliveries are required for domains that match queue_smtp_domains, they are not immediately delivered, but instead the message waits on the queue for the next queue run. Since routing of the message has taken place, Exim knows to which remote hosts it must be delivered, and so when the queue run happens, multiple messages for the same host are delivered over a single SMTP connection. The -odqs command line option causes all SMTP deliveries to be queued in this way, and is equivalent to setting queue_smtp_domains to “*”. See also hold_domains and queue_domains.

This option sets the timeout for accepting a non-SMTP message, that is, the maximum time that Exim waits when reading a message on the standard input. If the value is zero, it will wait for ever. This setting is overridden by the -or command line option. The timeout for incoming SMTP messages is controlled by smtp_receive_timeout.

This string defines the contents of the Received: message header that is added to each message, except for the timestamp, which is automatically added on at the end (preceded by a semicolon). The string is expanded each time it is used. If the expansion yields an empty string, no Received: header line is added to the message. Otherwise, the string should start with the text “Received:” and conform to the RFC 2822 specification for Received: header lines. The default setting is:

received_header_text = Received: \
  ${if def:sender_rcvhost {from $sender_rcvhost\n\t}\
  {${if def:sender_ident {from ${quote_local_part: $sender_ident} }}\
  ${if def:sender_helo_name {(helo=$sender_helo_name)\n\t}}}}\
  by $primary_hostname \
  ${if def:received_protocol {with $received_protocol}} \
  ${if def:tls_cipher {($tls_cipher)\n\t}}\
  (Exim $version_number)\n\t\
  ${if def:sender_address {(envelope-from <$sender_address>)\n\t}}\
  id $message_exim_id\
  ${if def:received_for {\n\tfor $received_for}}

The reference to the TLS cipher is omitted when Exim is built without TLS support. The use of conditional expansions ensures that this works for both locally generated messages and messages received from remote hosts, giving header lines such as the following:

Received: from scrooge.carol.example ([192.168.12.25] ident=root)
        by marley.carol.example with esmtp (Exim 4.00)
        (envelope-from <bob@carol.example>)
        id 16IOWa-00019l-00
        for chas@dickens.example; Tue, 25 Dec 2001 14:43:44 +0000
Received: by scrooge.carol.example with local (Exim 4.00)
        id 16IOWW-000083-00; Tue, 25 Dec 2001 14:43:41 +0000

Until the body of the message has been received, the timestamp is the time when the message started to be received. Once the body has arrived, and all policy checks have taken place, the timestamp is updated to the time at which the message was accepted.

When a message is to be delivered, the number of Received: headers is counted, and if it is greater than this parameter, a mail loop is assumed to have occurred, the delivery is abandoned, and an error message is generated. This applies to both local and remote deliveries.

This option lists those hosts from which Exim is prepared to accept unqualified recipient addresses in message envelopes. The addresses are made fully qualified by the addition of the qualify_recipient value. This option also affects message header lines. Exim does not reject unqualified recipient addresses in headers, but it qualifies them only if the message came from a host that matches recipient_unqualified_hosts, or if the message was submitted locally (not using TCP/IP), and the -bnq option was not set.

If this option is set greater than zero, it specifies the maximum number of original recipients for any message. Additional recipients that are generated by aliasing or forwarding do not count. SMTP messages get a 452 response for all recipients over the limit; earlier recipients are delivered as normal. Non-SMTP messages with too many recipients are failed, and no deliveries are done.

Note that the RFCs specify that an SMTP server should accept at least 100 RCPT commands in a single message.

If this option is set true, Exim rejects SMTP messages containing too many recipients by giving 552 errors to the surplus RCPT commands, and a 554 error to the eventual DATA command. Otherwise (the default) it gives a 452 error to the surplus RCPT commands and accepts the message on behalf of the initial set of recipients. The remote server should then re-send the message for the remaining recipients at a later time.

This option controls parallel delivery of one message to a number of remote hosts. If the value is less than 2, parallel delivery is disabled, and Exim does all the remote deliveries for a message one by one. Otherwise, if a single message has to be delivered to more than one remote host, or if several copies have to be sent to the same remote host, up to remote_max_parallel deliveries are done simultaneously. If more than remote_max_parallel deliveries are required, the maximum number of processes are started, and as each one finishes, another is begun. The order of starting processes is the same as if sequential delivery were being done, and can be controlled by the remote_sort_domains option. If parallel delivery takes place while running with debugging turned on, the debugging output from each delivery process is tagged with its process id.

This option controls only the maximum number of parallel deliveries for one message in one Exim delivery process. Because Exim has no central queue manager, there is no way of controlling the total number of simultaneous deliveries if the configuration allows a delivery attempt as soon as a message is received.

If you want to control the total number of deliveries on the system, you need to set the queue_only option. This ensures that all incoming messages are added to the queue without starting a delivery process. Then set up an Exim daemon to start queue runner processes at appropriate intervals (probably fairly often, for example, every minute), and limit the total number of queue runners by setting the queue_run_max parameter. Because each queue runner delivers only one message at a time, the maximum number of deliveries that can then take place at once is queue_run_max multiplied by remote_max_parallel.

If it is purely remote deliveries you want to control, use queue_smtp_domains instead of queue_only. This has the added benefit of doing the SMTP routing before queuing, so that several messages for the same host will eventually get delivered down the same connection.

When there are a number of remote deliveries for a message, they are sorted by domain into the order given by this list. For example,

remote_sort_domains = *.cam.ac.uk:*.uk

would attempt to deliver to all addresses in the cam.ac.uk domain first, then to those in the uk domain, then to any others.

This option sets a “use before” time on retry information in Exim’s hints database. Any older retry data is ignored. This means that, for example, once a host has not been tried for 7 days, Exim behaves as if it has no knowledge of past failures.

Chapter 32 describes Exim’s mechanisms for controlling the intervals between delivery attempts for messages that cannot be delivered straight away. This option sets an overall limit to the length of time between retries.

RFC 2821, section 4.4, states that an SMTP server must insert a Return-path: header line into a message when it makes a “final delivery”. The Return-path: header preserves the sender address as received in the MAIL command. This description implies that this header should not be present in an incoming message. If return_path_remove is true, any existing Return-path: headers are removed from messages at the time they are received. Exim’s transports have options for adding Return-path: headers at the time of delivery. They are normally used only for final local deliveries.

This option is an obsolete synonym for bounce_return_size_limit.

RFC 1413 identification calls are made to any client host which matches an item in the list.

This sets the timeout on RFC 1413 identification calls. If it is set to zero, no RFC 1413 calls are ever made.

This option lists those hosts from which Exim is prepared to accept unqualified sender addresses. The addresses are made fully qualified by the addition of qualify_domain. This option also affects message header lines. Exim does not reject unqualified addresses in headers that contain sender addresses, but it qualifies them only if the message came from a host that matches sender_unqualified_hosts, or if the message was submitted locally (not using TCP/IP), and the -bnq option was not set.

This option controls the setting of the SO_KEEPALIVE option on incoming TCP/IP socket connections. When set, it causes the kernel to probe idle connections periodically, by sending packets with “old” sequence numbers. The other end of the connection should send an acknowledgement if the connection is still okay or a reset if the connection has been aborted. The reason for doing this is that it has the beneficial effect of freeing up certain types of connection that can get stuck when the remote host is disconnected without tidying up the TCP/IP call properly. The keepalive mechanism takes several hours to detect unreachable hosts.

This option specifies the maximum number of simultaneous incoming SMTP calls that Exim will accept. It applies only to the listening daemon; there is no control (in Exim) when incoming SMTP is being handled by inetd. If the value is set to zero, no limit is applied. However, it is required to be non-zero if either smtp_accept_max_per_host or smtp_accept_queue is set. See also smtp_accept_reserve.

Exim counts the number of “non-mail” commands in an SMTP session, and drops the connection if there are too many. This option defines “too many”. The check catches some denial-of-service attacks, repeated failing AUTHs, or a mad client looping sending EHLO, for example. The check is applied only if the client host matches smtp_accept_max_nonmail_hosts.

When a new message is expected, one occurrence of RSET is not counted. This allows a client to send one RSET between messages (this is not necessary, but some clients do it). Exim also allows one uncounted occurence of HELO or EHLO, and one occurrence of STARTTLS between messages. After starting up a TLS session, another EHLO is expected, and so it too is not counted. The first occurrence of AUTH in a connection, or immediately following STARTTLS is not counted. Otherwise, all commands other than MAIL, RCPT, DATA, and QUIT are counted.

You can control which hosts are subject to the smtp_accept_max_nonmail check by setting this option. The default value makes it apply to all hosts. By changing the value, you can exclude any badly-behaved hosts that you have to live with.

The value of this option limits the number of MAIL commands that Exim is prepared to accept over a single SMTP connection, whether or not each command results in the transfer of a message. After the limit is reached, a 421 response is given to subsequent MAIL commands. This limit is a safety precaution against a client that goes mad (incidents of this type have been seen).

This option restricts the number of simultaneous IP connections from a single host (strictly, from a single IP address) to the Exim daemon. The option is expanded, to enable different limits to be applied to different hosts by reference to $sender_host_address. Once the limit is reached, additional connection attempts from the same host are rejected with error code 421. The default value of zero imposes no limit. If this option is set, it is required that smtp_accept_max be non-zero.

Warning: When setting this option you should not use any expansion constructions that take an appreciable amount of time. The expansion and test happen in the main daemon loop, in order to reject additional connections without forking additional processes (otherwise a denial-of-service attack could cause a vast number or processes to be created). While the daemon is doing this processing, it cannot accept any other incoming connections.

If the number of simultaneous incoming SMTP calls handled via the listening daemon exceeds this value, messages received by SMTP are just placed on the queue; no delivery processes are started automatically. A value of zero implies no limit, and clearly any non-zero value is useful only if it is less than the smtp_accept_max value (unless that is zero). See also queue_only, queue_only_load, queue_smtp_domains, and the various -od command line options.

This option limits the number of delivery processes that Exim starts automatically when receiving messages via SMTP, whether via the daemon or by the use of -bs or -bS. If the value of the option is greater than zero, and the number of messages received in a single SMTP session exceeds this number, subsequent messages are placed on the queue, but no delivery processes are started. This helps to limit the number of Exim processes when a server restarts after downtime and there is a lot of mail waiting for it on other systems. On large systems, the default should probably be increased, and on dial-in client systems it should probably be set to zero (that is, disabled).

When smtp_accept_max is set greater than zero, this option specifies a number of SMTP connections that are reserved for connections from the hosts that are specified in smtp_reserve_hosts. The value set in smtp_accept_max includes this reserve pool. The specified hosts are not restricted to this number of connections; the option specifies a minimum number of connection slots for them, not a maximum. It is a guarantee that that group of hosts can always get at least smtp_accept_reserve connections.

For example, if smtp_accept_max is set to 50 and smtp_accept_reserve is set to 5, once there are 45 active connections (from any hosts), new connections are accepted only from hosts listed in smtp_reserve_hosts. See also smtp_accept_max_per_host.

This option is provided for multi-homed servers that want to masquerade as several different hosts. At the start of an SMTP connection, its value is expanded and used instead of the value of $primary_hostname in SMTP responses. For example, it is used as domain name in the response to an incoming HELO or EHLO command.

It is also used in HELO commands for callout verification. The active hostname is placed in the $smtp_active_hostname variable, which is saved with any messages that are received. It is therefore available for use in routers and transports when the message is later delivered.

If this option is unset, or if its expansion is forced to fail, or if the expansion results in an empty string, the value of $primary_hostname is used. Other expansion failures cause a message to be written to the main and panic logs, and the SMTP command receives a temporary error. Typically, the value of smtp_active_hostname depends on the incoming interface address. For example:

smtp_active_hostname = ${if eq{$interface_address}{10.0.0.1}\
  {cox.mydomain}{box.mydomain}}

This string, which is expanded every time it is used, is output as the initial positive response to an SMTP connection. The default setting is:

smtp_banner = $smtp_active_hostname ESMTP Exim \
  $version_number $tod_full

Failure to expand the string causes a panic error. If you want to create a multiline response to the initial SMTP connection, use “\n” in the string at appropriate points, but not at the end. Note that the 220 code is not included in this string. Exim adds it automatically (several times in the case of a multiline response).

When this option is set, if an incoming SMTP session encounters the SIZE option on a MAIL command, it checks that there is enough space in the spool directory’s partition to accept a message of that size, while still leaving free the amount specified by check_spool_space (even if that value is zero). If there isn’t enough space, a temporary error code is returned.

This option specifies a maximum number of waiting SMTP connections. Exim passes this value to the TCP/IP system when it sets up its listener. Once this number of connections are waiting for the daemon’s attention, subsequent connection attempts are refused at the TCP/IP level. At least, that is what the manuals say; in some circumstances such connection attempts have been observed to time out instead. For large systems it is probably a good idea to increase the value (to 50, say). It also gives some protection against denial-of-service attacks by SYN flooding.

The SMTP protocol specification requires the client to wait for a response from the server at certain points in the dialogue. Without PIPELINING these synchronization points are after every command; with PIPELINING they are fewer, but they still exist.

Some spamming sites send out a complete set of SMTP commands without waiting for any response. Exim protects against this by rejecting a message if the client has sent further input when it should not have. The error response “554 SMTP synchronization error” is sent, and the connection is dropped. Testing for this error cannot be perfect because of transmission delays (unexpected input may be on its way but not yet received when Exim checks). However, it does detect many instances.

The check can be globally disabled by setting smtp_enforce_sync false. If you want to disable the check selectively (for example, only for certain hosts), you can do so by an appropriate use of a control modifier in an ACL (see section 39.18). See also pipelining_advertise_hosts.

If this option is set, the given command is run whenever an SMTP ETRN command is received from a host that is permitted to issue such commands (see chapter 39). The string is split up into separate arguments which are independently expanded. The expansion variable $domain is set to the argument of the ETRN command, and no syntax checking is done on it. For example:

smtp_etrn_command = /etc/etrn_command $domain $sender_host_address

A new process is created to run the command, but Exim does not wait for it to complete. Consequently, its status cannot be checked. If the command cannot be run, a line is written to the panic log, but the ETRN caller still receives a 250 success response. Exim is normally running under its own uid when receiving SMTP, so it is not possible for it to change the uid before running the command.

When this option is set, it prevents the simultaneous execution of more than one identical command as a result of ETRN in an SMTP connection. See section 44.9 for details.

If the system load average ever gets higher than this, incoming SMTP calls are accepted only from those hosts that match an entry in smtp_reserve_hosts. If smtp_reserve_hosts is not set, no incoming SMTP calls are accepted when the load is over the limit. The option has no effect on ancient operating systems on which Exim cannot determine the load average. See also deliver_queue_load_max and queue_only_load.

Exim rejects SMTP commands that contain syntax or protocol errors. In particular, a syntactically invalid email address, as in this command:

RCPT TO:<abc xyz@a.b.c>

causes immediate rejection of the command, before any other tests are done. (The ACL cannot be run if there is no valid address to set up for it.) An example of a protocol error is receiving RCPT before MAIL. If there are too many syntax or protocol errors in one SMTP session, the connection is dropped. The limit is set by this option.

When the PIPELINING extension to SMTP is in use, some protocol errors are “expected”, for instance, a RCPT command after a rejected MAIL command. Exim assumes that PIPELINING will be used if it advertises it (see pipelining_advertise_hosts), and in this situation, “expected” errors do not count towards the limit.

If there are too many unrecognized commands in an incoming SMTP session, an Exim server drops the connection. This is a defence against some kinds of abuse that subvert web clients into making connections to SMTP ports; in these circumstances, a number of non-SMTP command lines are sent first.

Some sites find it helpful to be able to limit the rate at which certain hosts can send them messages, and the rate at which an individual message can specify recipients. When a host matches smtp_ratelimit_hosts, the values of smtp_ratelimit_mail and smtp_ratelimit_rcpt are used to control the rate of acceptance of MAIL and RCPT commands in a single SMTP session, respectively. Each option, if set, must contain a set of four comma-separated values:

For example, these settings have been used successfully at the site which first suggested this feature, for controlling mail from their customers:

smtp_ratelimit_mail = 2,0.5s,1.05,4m
smtp_ratelimit_rcpt = 4,0.25s,1.015,4m

The first setting specifies delays that are applied to MAIL commands after two have been received over a single connection. The initial delay is 0.5 seconds, increasing by a factor of 1.05 each time. The second setting applies delays to RCPT commands when more than four occur in a single message.

It is also possible to configure delays explicitly in ACLs. See section 39.17 for details.

See smtp_ratelimit_hosts above.

See smtp_ratelimit_hosts above.

This sets a timeout value for SMTP reception. It applies to all forms of SMTP input, including batch SMTP. If a line of input (either an SMTP command or a data line) is not received within this time, the SMTP connection is dropped and the message is abandoned. A line is written to the log containing one of the following messages:

SMTP command timeout on connection from...
SMTP data timeout on connection from...

The former means that Exim was expecting to read an SMTP command; the latter means that it was in the DATA phase, reading the contents of a message.

The value set by this option can be overridden by the -os command-line option. A setting of zero time disables the timeout, but this should never be used for SMTP over TCP/IP. (It can be useful in some cases of local input using -bs or -bS.) For non-SMTP input, the reception timeout is controlled by receive_timeout and -or.

This option defines hosts for which SMTP connections are reserved; see smtp_accept_reserve and smtp_load_reserve above.

In the default state, Exim uses bland messages such as “Administrative prohibition” when it rejects SMTP commands for policy reasons. Many sysadmins like this because it gives away little information to spammers. However, some other syadmins who are applying strict checking policies want to give out much fuller information about failures. Setting smtp_return_error_details true causes Exim to be more forthcoming. For example, instead of “Administrative prohibition”, it might give:

550-Rejected after DATA: '>' missing at end of address:
550 failing address in "From" header is: <user@dom.ain

This option is available when Exim is compiled with the content-scanning extension. It specifies how Exim connects to SpamAssassin’s spamd daemon. See section 40.2 for more details.

If this option is set, it causes Exim to split its input directory into 62 subdirectories, each with a single alphanumeric character as its name. The sixth character of the message id is used to allocate messages to subdirectories; this is the least significant base-62 digit of the time of arrival of the message.

Splitting up the spool in this way may provide better performance on systems where there are long mail queues, by reducing the number of files in any one directory. The msglog directory is also split up in a similar way to the input directory; however, if preserve_message_logs is set, all old msglog files are still placed in the single directory msglog.OLD.

It is not necessary to take any special action for existing messages when changing split_spool_directory. Exim notices messages that are in the “wrong” place, and continues to process them. If the option is turned off after a period of being on, the subdirectories will eventually empty and be automatically deleted.

When split_spool_directory is set, the behaviour of queue runner processes changes. Instead of creating a list of all messages in the queue, and then trying to deliver each one in turn, it constructs a list of those in one sub-directory and tries to deliver them, before moving on to the next sub-directory. The sub-directories are processed in a random order. This spreads out the scanning of the input directories, and uses less memory. It is particularly beneficial when there are lots of messages on the queue. However, if queue_run_in_order is set, none of this new processing happens. The entire queue has to be scanned and sorted before any deliveries can start.

This defines the directory in which Exim keeps its spool, that is, the messages it is waiting to deliver. The default value is taken from the compile-time configuration setting, if there is one. If not, this option must be set. The string is expanded, so it can contain, for example, a reference to $primary_hostname.

If the spool directory name is fixed on your installation, it is recommended that you set it at build time rather than from this option, particularly if the log files are being written to the spool directory (see log_file_path). Otherwise log files cannot be used for errors that are detected early on, such as failures in the configuration file.

By using this option to override the compiled-in path, it is possible to run tests of Exim without using the standard spool.

If this option is set, redundant pairs of angle brackets round “route-addr” items in addresses are stripped. For example, \<\<xxx@a.b.c.d\>\> is treated as <xxx@a.b.c.d>. If this is in the envelope and the message is passed on to another MTA, the excess angle brackets are not passed on. If this option is not set, multiple pairs of angle brackets cause a syntax error.

If this option is set, a trailing dot at the end of a domain in an address is ignored. If this is in the envelope and the message is passed on to another MTA, the dot is not passed on. If this option is not set, a dot at the end of a domain causes a syntax error. However, addresses in header lines are checked only when an ACL requests header syntax checking.

When Exim is logging to syslog, it writes the log lines for its three separate logs at different syslog priorities so that they can in principle be separated on the logging hosts. Some installations do not require this separation, and in those cases, the duplication of certain log lines is a nuisance. If syslog_duplication is set false, only one copy of any particular log line is written to syslog. For lines that normally go to both the main log and the reject log, the reject log version (possibly containing message header lines) is written, at LOG_NOTICE priority. Lines that normally go to both the main and the panic log are written at the LOG_ALERT priority.

This option sets the syslog “facility” name, used when Exim is logging to syslog. The value must be one of the strings “mail”, “user”, “news”, “uucp”, “daemon”, or “local’x'” where x is a digit between 0 and 7. If this option is unset, “mail” is used. See chapter 48 for details of Exim’s logging.

This option sets the syslog “ident” name, used when Exim is logging to syslog. The value must be no longer than 32 characters. See chapter 48 for details of Exim’s logging.

If syslog_timestamp is set false, the timestamps on Exim’s log lines are omitted when these lines are sent to syslog. See chapter 48 for details of Exim’s logging.

This option specifies an Exim filter file that is applied to all messages at the start of each delivery attempt, before any routing is done. System filters must be Exim filters; they cannot be Sieve filters. If the system filter generates any deliveries to files or pipes, or any new mail messages, the appropriate system_filter_…_transport option(s) must be set, to define which transports are to be used. Details of this facility are given in chapter 42.

This sets the name of the transport driver that is to be used when the save command in a system message filter specifies a path ending in “/”, implying delivery of each message into a separate file in some directory. During the delivery, the variable $address_file contains the path name.

This sets the name of the transport driver that is to be used when the save command in a system message filter specifies a path not ending in “/”. During the delivery, the variable $address_file contains the path name.

This option is used only when system_filter_user is also set. It sets the gid under which the system filter is run, overriding any gid that is associated with the user. The value may be numerical or symbolic.

This specifies the transport driver that is to be used when a pipe command is used in a system filter. During the delivery, the variable $address_pipe contains the pipe command.

This specifies the transport driver that is to be used when a mail command is used in a system filter.

If this option is not set, the system filter is run in the main Exim delivery process, as root. When the option is set, the system filter runs in a separate process, as the given user. Unless the string consists entirely of digits, it is looked up in the password data. Failure to find the named user causes a configuration error. The gid is either taken from the password data, or specified by system_filter_group. When the uid is specified numerically, system_filter_group is required to be set.

If the system filter generates any pipe, file, or reply deliveries, the uid under which the filter is run is used when transporting them, unless a transport option overrides. Normally you should set system_filter_user if your system filter generates these kinds of delivery.

If this option is set false, it stops the Exim daemon setting the TCP_NODELAY option on its listening sockets. Setting TCP_NODELAY turns off the “Nagle algorithm”, which is a way of improving network performance in interactive (character-by-character) situations. Turning it off should improve Exim’s performance a bit, so that is what happens by default. However, it appears that some broken clients cannot cope, and time out. Hence this option. It affects only those sockets that are set up for listening by the daemon. Sockets created by the smtp transport for delivering mail always set TCP_NODELAY.

If timeout_frozen_after is set to a time greater than zero, a frozen message of any kind that has been on the queue for longer than the given time is automatically cancelled at the next queue run. If it is a bounce message, it is just discarded; otherwise, a bounce is sent to the sender, in a similar manner to cancellation by the -Mg command line option. If you want to timeout frozen bounce messages earlier than other kinds of frozen message, see ignore_bounce_errors_after.

The value of timezone is used to set the environment variable TZ while running Exim (if it is different on entry). This ensures that all timestamps created by Exim are in the required timezone. If you want all your timestamps to be in UTC (aka GMT) you should set

timezone = UTC

The default value is taken from TIMEZONE_DEFAULT in Local/Makefile, or, if that is not set, from the value of the TZ environment variable when Exim is built. If timezone is set to the empty string, either at build or run time, any existing TZ variable is removed from the environment when Exim runs. This is appropriate behaviour for obtaining wall-clock time on some, but unfortunately not all, operating systems.

When Exim is built with support for TLS encrypted connections, the availability of the STARTTLS command to set up an encrypted session is advertised in response to EHLO only to those client hosts that match this option. See chapter 38 for details of Exim’s support for TLS.

The value of this option is expanded, and must then be the absolute path to a file which contains the server’s certificates. The server’s private key is also assumed to be in this file if tls_privatekey is unset. See chapter 38 for further details.

Note: The certificates defined by this option are used only when Exim is receiving incoming messages as a server. If you want to supply certificates for use when sending messages as a client, you must set the tls_certificate option in the relevant smtp transport.

This option specifies a certificate revocation list. The expanded value must be the name of a file that contains a CRL in PEM format.

The value of this option is expanded, and must then be the absolute path to a file which contains the server’s DH parameter values. This is used only for OpenSSL. When Exim is linked with GnuTLS, this option is ignored. See section 38.2 for further details.

This option specifies a list of incoming SSMTP (aka SMTPS) ports that should operate the obsolete SSMTP (SMTPS) protocol, where a TLS session is immediately set up without waiting for the client to issue a STARTTLS command. For further details, see section 13.4.

The value of this option is expanded, and must then be the absolute path to a file which contains the server’s private key. If this option is unset, the private key is assumed to be in the same file as the server’s certificates. See chapter 38 for further details.

If this option is set true, Exim violates the RFCs by remembering that it is in “esmtp” state after successfully negotiating a TLS session. This provides support for broken clients that fail to send a new EHLO after starting a TLS session.

This option controls which ciphers can be used for incoming TLS connections. The smtp transport has an option of the same name for controlling outgoing connections. This option is expanded for each connection, so can be varied for different clients if required. The value of this option must be a list of permitted cipher suites. The OpenSSL and GnuTLS libraries handle cipher control in somewhat different ways.

If GnuTLS is being used, the client controls the preference order of the available ciphers.

Details are given in sections 38.4 and 38.5.

See tls_verify_hosts below.

The value of this option is expanded, and must then be the absolute path to a file containing permitted certificates for clients that match tls_verify_hosts or tls_try_verify_hosts. Alternatively, if you are using OpenSSL, you can set tls_verify_certificates to the name of a directory containing certificate files. This does not work with GnuTLS; the option must be set to the name of a single file if you are using GnuTLS.

This option, along with tls_try_verify_hosts, controls the checking of certificates from clients. The expected certificates are defined by tls_verify_certificates, which must be set. A configuration error occurs if either tls_verify_hosts or tls_try_verify_hosts is set and tls_verify_certificates is not set.

Any client that matches tls_verify_hosts is constrained by tls_verify_certificates. The client must present one of the listed certificates. If it does not, the connection is aborted.

A weaker form of checking is provided by tls_try_verify_hosts. If a client matches this option (but not tls_verify_hosts), Exim requests a certificate and checks it against tls_verify_certificates, but does not abort the connection if there is no certificate or if it does not match. This state can be detected in an ACL, which makes it possible to implement policies such as “accept for relay only if a verified certificate has been received, but accept for local delivery if encrypted, even without a verified certificate”.

Client hosts that match neither of these lists are not asked to present certificates.

This is a specialized feature for use in unusual configurations. By default, if the uid of the caller of Exim cannot be looked up using getpwuid(), Exim gives up. The unknown_login option can be used to set a login name to be used in this circumstance. It is expanded, so values like user$caller_uid can be set. When unknown_login is used, the value of unknown_username is used for the user’s real name (gecos field), unless this has been set by the -F option.