The -bh command line option provides a way of testing your ACL configuration locally by running a fake SMTP session with which you interact. The host relay-test.mail-abuse.org provides a service for checking your relaying configuration (see section 39.40 for more details).

In order to cause an ACL to be used, you have to name it in one of the relevant options in the main part of the configuration. These options are:

For example, if you set

acl_smtp_rcpt = small_acl

the little ACL defined above is used whenever Exim receives a RCPT command in an SMTP dialogue. The majority of policy tests on incoming messages can be done when RCPT commands arrive. A rejection of RCPT should cause the sending MTA to give up on the recipient address contained in the RCPT command, whereas rejection at other times may cause the client MTA to keep on trying to deliver the message. It is therefore recommended that you do as much testing as possible at RCPT time.

The non-SMTP ACL applies to all non-interactive incoming messages, that is, it applies to batch SMTP as well as to non-SMTP messages. (Batch SMTP is not really SMTP.) This ACL is run just before the local_scan() function. Any kind of rejection is treated as permanent, because there is no way of sending a temporary error for these kinds of message. Many of the ACL conditions (for example, host tests, and tests on the state of the SMTP connection such as encryption and authentication) are not relevant and are forbidden in this ACL.

The ACL test specified by acl_smtp_connect happens after the test specified by host_reject_connection (which is now an anomaly) and any TCP Wrappers testing (if configured).

Two ACLs are associated with the DATA command, because it is two-stage command, with two responses being sent to the client. When the DATA command is received, the ACL defined by acl_smtp_predata is obeyed. This gives you control after all the RCPT commands, but before the message itself is received. It offers the opportunity to give a negative response to the DATA command before the data is transmitted. Header lines added by MAIL or RCPT ACLs are not visible at this time, but any that are defined here are visible when the acl_smtp_data ACL is run.

You cannot test the contents of the message, for example, to verify addresses in the headers, at RCPT time or when the DATA command is received. Such tests have to appear in the ACL that is run after the message itself has been received, before the final response to the DATA command is sent. This is the ACL specified by acl_smtp_data, which is the second ACL that is associated with the DATA command.

For both of these ACLs, it is not possible to reject individual recipients. An error response rejects the entire message. Unfortunately, it is known that some MTAs do not treat hard (5xx) responses to the DATA command (either before or after the data) correctly – they keep the message on their queues and try again later, but that is their problem, though it does waste some of your resources.

The acl_smtp_mime option is available only when Exim is compiled with the content-scanning extension. For details, see chapter 40.

The ACL for the SMTP QUIT command is anomalous, in that the outcome of the ACL does not affect the response code to QUIT, which is always 221. Thus, the ACL does not in fact control any access. For this reason, the only verbs that are permitted are accept and warn.

This ACL can be used for tasks such as custom logging at the end of an SMTP session. For example, you can use ACL variables in other ACLs to count messages, recipients, etc., and log the totals at QUIT time using one or more logwrite modifiers on a warn verb.

You do not need to have a final accept, but if you do, you can use a message modifier to specify custom text that is sent as part of the 221 response to QUIT.

This ACL is run only for a “normal” QUIT. For certain kinds of disastrous failure (for example, failure to open a log file, or when Exim is bombing out because it has detected an unrecoverable error), all SMTP commands from the client are given temporary error responses until QUIT is received or the connection is closed. In these special cases, the QUIT ACL does not run.

The value of an acl_smtp_'xxx' option is expanded before use, so you can use different ACLs in different circumstances. The resulting string does not have to be the name of an ACL in the configuration file; there are other possibilities. Having expanded the string, Exim searches for an ACL as follows:

Except for the QUIT ACL, which does not affect the SMTP return code (see section 39.7 above), the

result of running an ACL is either “accept” or “deny”, or, if some test cannot be completed (for example, if a database is down), “defer”. These results cause 2xx, 5xx, and 4xx return codes, respectively, to be used in the SMTP dialogue. A fourth return, “error”, occurs when there is an error such as invalid syntax in the ACL. This also causes a 4’xx’ return code.

For the non-SMTP ACL, “defer” and “error” are treated in the same way as “deny”, because there is no mechanism for passing temporary errors to the submitters of non-SMTP messages.

ACLs that are relevant to message reception may also return “discard”. This has the effect of “accept”, but causes either the entire message or an individual recipient address to be discarded. In other words, it is a blackholing facility. Use it with care.

If the ACL for MAIL returns “discard”, all recipients are discarded, and no ACL is run for subsequent RCPT commands. The effect of “discard” in a RCPT ACL is to discard just the one recipient address. If there are no recipients left when the message’s data is received, the DATA ACL is not run. A “discard” return from the DATA or the non-SMTP ACL discards all the remaining recipients.

The “discard” return is not permitted for the acl_smtp_predata ACL.

The local_scan() function is always run, even if there are no remaining recipients; it may create new recipients.

The default actions when any of the acl_'xxx' options are unset are not all the same. Note: These defaults apply only when the relevant ACL is not defined at all. For any defined ACL, the default action when control reaches the end of the ACL statements is “deny”.

For acl_not_smtp, acl_smtp_auth, acl_smtp_connect, acl_smtp_data, acl_smtp_helo, acl_smtp_mail, acl_smtp_mailauth, acl_smtp_mime, acl_smtp_predata, acl_smtp_quit, and acl_smtp_starttls, the action when the ACL is not defined is “accept”.

For the others (acl_smtp_etrn, acl_smtp_expn, acl_smtp_rcpt, and acl_smtp_vrfy), the action when the ACL is not defined is “deny”. This means that acl_smtp_rcpt must be defined in order to receive any messages over an SMTP connection. For an example, see the ACL in the default configuration file.

When a MAIL or RCPT ACL, or either of the DATA ACLs, is running, the variables that contain information about the host and the message’s sender (for example, $sender_host_address and $sender_address) are set, and can be used in ACL statements. In the case of RCPT (but not MAIL or DATA), $domain and $local_part are set from the argument address.

When an ACL for the AUTH parameter of MAIL is running, the variables that contain information about the host are set, but $sender_address is not yet set. Section 33.2 contains a discussion of this parameter and how it is used.

The $message_size variable is set to the value of the SIZE parameter on the MAIL command at MAIL, RCPT and pre-data time, or to -1 if that parameter is not given. The value is updated to the true message size by the time the final DATA ACL is run (after the message data has been received).

The $rcpt_count variable increases by one for each RCPT command received. The $recipients_count variable increases by one each time a RCPT command is accepted, so while an ACL for RCPT is being processed, it contains the number of previously accepted recipients. At DATA time (for both the DATA ACLs), $rcpt_count contains the total number of RCPT commands, and $recipients_count contains the total number of accepted recipients.

When an ACL is being run for AUTH, EHLO, ETRN, EXPN, HELO, STARTTLS, or VRFY, the remainder of the SMTP command line is placed in $smtp_command_argument. This can be tested using a condition condition. For example, here is an ACL for use with AUTH, which insists that either the session is encrypted, or the CRAM-MD5 authentication method is used. In other words, it does not permit authentication methods that use cleartext passwords on unencrypted connections.

acl_check_auth:
  accept encrypted = *
  accept condition = ${if eq{${uc:$smtp_command_argument}}\
                     {CRAM-MD5}}
  deny   message   = TLS encryption or CRAM-MD5 required

(Another way of applying this restriction is to arrange for the authenticators that use cleartext passwords not to be advertised when the connection is not encrypted. You can use the generic server_advertise_condition authenticator option to do this.)

An individual ACL consists of a number of statements. Each statement starts with a verb, optionally followed by a number of conditions and “modifiers”. Modifiers can change the way the verb operates, define error and log messages, set variables, insert delays, and vary the processing of accepted messages.

If all the conditions are met, the verb is obeyed. The same condition may be used (with different arguments) more than once in the same statement. This provides a means of specifying an “and” conjunction between conditions. For example:

deny  dnslists = list1.example
      dnslists = list2.example

If there are no conditions, the verb is always obeyed. Exim stops evaluating the conditions and modifiers when it reaches a condition that fails. What happens then depends on the verb (and in one case, on a special modifier). Not all the conditions make sense at every testing point. For example, you cannot test a sender address in the ACL that is run for a VRFY command.

The ACL verbs are as follows:

At the end of each ACL there is an implicit unconditional deny.

As you can see from the examples above, the conditions and modifiers are written one to a line, with the first one on the same line as the verb, and subsequent ones on following lines. If you have a very long condition, you can continue it onto several physical lines by the usual backslash continuation mechanism. It is conventional to align the conditions vertically.

There are some special variables that can be set during ACL processing. They can be used to pass information between different ACLs, different invocations of the same ACL in the same SMTP connection, and between ACLs and the routers, transports, and filters that are used to deliver a message. There are two sets of these variables:

When a message is accepted, the current values of all the ACL variables are preserved with the message and are subsequently made available at delivery time. The ACL variables are set by modifier called set. For example:

accept hosts = whatever
       set acl_m4 = some value

Note: a leading dollar sign is not used when naming a variable that is to be set. If you want to set a variable without taking any action, you can use a warn verb without any other modifiers or conditions.

An exclamation mark preceding a condition negates its result. For example:

deny   domains = *.dom.example
      !verify = recipient

deny  hosts = !192.168.3.4
deny !hosts =  192.168.3.4

The arguments of conditions and modifiers are expanded. A forced failure of an expansion causes a condition to be ignored, that is, it behaves as if the condition is true. Consider these two statements:

accept  senders = ${lookup{$host_name}lsearch\
                  {/some/file}{$value}fail}
accept  senders = ${lookup{$host_name}lsearch\
                  {/some/file}{$value}{}}

Each attempts to look up a list of acceptable senders. If the lookup succeeds, the returned list is searched, but if the lookup fails the behaviour is different in the two cases. The fail in the first statement causes the condition to be ignored, leaving no further conditions. The accept verb therefore succeeds. The second statement, however, generates an empty list when the lookup fails. No sender can match an empty list, so the condition fails, and therefore the accept also fails.

ACL modifiers appear mixed in with conditions in ACL statements. Some of them specify actions that are taken as the conditions for a statement are checked; others specify text for messages that are used when access is denied or a warning is generated. The control modifier affects the way an incoming message is handled.

The positioning of the modifiers in an ACL statement important, because the processing of a verb ceases as soon as its outcome is known. Only those modifiers that have already been encountered will take effect. For example, consider this use of the message modifier:

require message = Can't verify sender
        verify  = sender
        message = Can't verify recipient
        verify  = recipient
        message = This message cannot be used

If sender verification fails, Exim knows that the result of the statement is “deny”, so it goes no further. The first message modifier has been seen, so its text is used as the error message. If sender verification succeeds, but recipient verification fails, the second message is used. If recipient verification succeeds, the third message becomes “current”, but is never used because there are no more conditions to cause failure.

For the deny verb, on the other hand, it is always the last message modifier that is used, because all the conditions must be true for rejection to happen. Specifying more than one message modifier does not make sense, and the message can even be specified after all the conditions. For example:

deny   hosts = ...
      !senders = *@my.domain.example
       message = Invalid sender from client host

The “deny” result does not happen until the end of the statement is reached, by which time Exim has set up the message.

The ACL modifiers are as follows:

The control modifier supports the following settings:

The message modifier can be used on a warn statement to add an extra header line to an incoming message, as in this example:

warn message = X-blacklisted-at: $dnslist_domain
     dnslists = sbl.spamhaus.org : \
                dialup.mail-abuse.org

If an identical header line is requested several times (provoked, for example, by multiple RCPT commands), only one copy is actually added to the message. If the text of the message modifier contains one or more newlines that are not followed by a space or a tab, it is assumed to contain multiple header lines. Each one is checked for valid syntax; X-ACL-Warn: is added to the front of any line that is not a valid header line.

By default, new lines are added at the end of the existing header lines. However, you can specify that any particular header line should be added right at the start (before all the Received: lines), immediately after the first block of Received: lines, or immediately before any line that is not a Received: or Resent-something: header.

This is done by specifying “:at_start:”, “:after_received:”, or “:at_start_rfc:” (or, for completeness, “:at_end:”) before the text of the header line, respectively. (Header text cannot start with a colon, as there has to be a header name first.) For example:

warn message = :after_received:X-My-Header: something or other...

If more than one header is supplied in a single warn statement, each one is treated independently and can therefore be placed differently. If you add more than one line at the start, or after the Received: block, they will end up in reverse order.

Warning: This facility currently applies only to header lines that are added in an ACL. It does NOT work for header lines that are added in a system filter or in a router or transport.

If you want to preserve data between MAIL, RCPT, and the acl_smtp_predata ACLs, you can use ACL variables, as described in section 39.15.

Some of conditions listed in this section are available only when Exim is compiled with the content-scanning extension. They are included here briefly for completeness. More detailed descriptions can be found in the discussion on content scanning in chapter 40.

Not all conditions are relevant in all circumstances. For example, testing senders and recipients does not make sense in an ACL that is being run as the result of the arrival of an ETRN command, and checks on message headers can be done only in the ACLs specified by acl_smtp_data and acl_not_smtp. You can use the same condition (with different parameters) more than once in the same ACL statement. This provides a way of specifying an “and” conjunction. The conditions are as follows:

In its simplest form, the dnslists condition tests whether the calling host is on at least one of a number of DNS lists by looking up the inverted IP address in one or more DNS domains. For example, if the calling host’s IP address is 192.168.62.43, and the ACL statement is

deny dnslists = blackholes.mail-abuse.org : \
                dialups.mail-abuse.org

the following records are looked up:

43.62.168.192.blackholes.mail-abuse.org
43.62.168.192.dialups.mail-abuse.org

As soon as Exim finds an existing DNS record, processing of the list stops. Thus, multiple entries on the list provide an “or” conjunction. If you want to test that a host is on more than one list (an “and” conjunction), you can use two separate conditions:

deny dnslists = blackholes.mail-abuse.org
     dnslists = dialups.mail-abuse.org

If a DNS lookup times out or otherwise fails to give a decisive answer, Exim behaves as if the host does not match the list item, that is, as if the DNS record does not exist. If there are further items in the DNS list, they are processed.

This is usually the required action when dnslists is used with deny (which is the most common usage), because it prevents a DNS failure from blocking mail. However, you can change this behaviour by putting one of the following special items in the list:

Each of these applies to any subsequent items on the list. For example:

deny dnslists = +defer_unknown : foo.bar.example

Testing the list of domains stops as soon as a match is found. If you want to warn for one list and block for another, you can use two different statements:

deny  dnslists = blackholes.mail-abuse.org
warn  message  = X-Warn: sending host is on dialups list
      dnslists = dialups.mail-abuse.org

DNS list lookups are cached by Exim for the duration of the SMTP session, so a lookup based on the IP address is done at most once for any incoming connection. Exim does not share information between multiple incoming connections (but your local name server cache should be active).

By default, the IP address that is used in a DNS list lookup is the IP address of the calling host. However, you can specify another IP address by listing it after the domain name, introduced by a slash. For example:

deny dnslists = black.list.tld/192.168.1.2

This feature is not very helpful with explicit IP addresses; it is intended for use with IP addresses that are looked up, for example, the IP addresses of the MX hosts or nameservers of an email sender address. For an example, see section 39.24 below.

There are some lists that are keyed on domain names rather than inverted IP addresses (see for example the domain based zones link at http://www.rfc-ignorant.org/). No reversing of components is used with these lists. You can change the name that is looked up in a DNS list by listing it after the domain name, introduced by a slash. For example,

deny  message  = Sender's domain is listed at $dnslist_domain
      dnslists = dsn.rfc-ignorant.org/$sender_address_domain

This particular example is useful only in ACLs that are obeyed after the RCPT or DATA commands, when a sender address is available. If (for example) the message’s sender is user@tld.example the name that is looked up by this example is

tld.example.dsn.rfc-ignorant.org

A single dnslists condition can contain entries for both names and IP addresses. For example:

deny dnslists = sbl.spamhaus.org : \
                dsn.rfc-ignorant.org/$sender_address_domain

The first item checks the sending host’s IP address; the second checks a domain name. The whole condition is true if either of the DNS lookups succeeds.

The syntax described above for looking up explicitly-defined values (either names or IP addresses) in a DNS blacklist is a simplification. After the domain name for the DNS list, what follows the slash can in fact be a list of items. As with all lists in Exim, the default separator is a colon. However, because this is a sublist within the list of DNS blacklist domains, it is necessary either to double the separators like this:

dnslists = black.list.tld/name.1::name.2

or to change the separator character, like this:

dnslists = black.list.tld/<;name.1;name.2

If an item in the list is an IP address, it is inverted before the DNS blacklist domain is appended. If it is not an IP address, no inversion occurs. Consider this condition:

dnslists = black.list.tld/<;192.168.1.2;a.domain

The DNS lookups that occur are:

2.1.168.192.black.list.tld
a.domain.black.list.tld

Once a DNS record has been found (that matches a specific IP return address, if specified – see section 39.27), no further lookups are done. If there is a temporary DNS error, the rest of the sublist of domains or IP addresses is tried. A temporary error for the whole dnslists item occurs only if no other DNS lookup in this sublist succeeds. In other words, a successful lookup for any of the items in the sublist overrides a temporary error for a previous item.

The ability to supply a list of items after the slash is in some sense just a syntactic convenience. These two examples have the same effect:

dnslists = black.list.tld/a.domain : black.list.tld/b.domain
dnslists = black.list.tld/a.domain::b.domain

However, when the data for the list is obtained from a lookup, the second form is usually much more convenient. Consider this example:

deny message  =  The mail servers for the domain \
                 $sender_address_domain \
                 are listed at $dnslist_domain ($dnslist_value); \
                 see $dnslist_text.
     dnslists =  sbl.spamhaus.org/<|${lookup dnsdb {>|a=<|\
                                    ${lookup dnsdb {>|mxh=\
                                    $sender_address_domain} }} }

Note the use of >| in the dnsdb lookup to specify the separator for multiple DNS records. The inner dnsdb lookup produces a list of MX hosts and the outer dnsdb lookup finds the IP addresses for these hosts. The result of expanding the condition might be something like this:

dnslists = sbl.spahmaus.org/<|192.168.2.3|192.168.5.6|...

Thus, this example checks whether or not the IP addresses of the sender domain’s mail servers are on the Spamhaus black list.

DNS lists are constructed using address records in the DNS. The original RBL just used the address 127.0.0.1 on the right hand side of each record, but the RBL+ list and some other lists use a number of values with different meanings. The values used on the RBL+ list are:

Some DNS lists may return more than one address record.

When an entry is found in a DNS list, the variable $dnslist_domain contains the name of the domain that matched, $dnslist_value contains the data from the entry, and $dnslist_text contains the contents of any associated TXT record. If more than one address record is returned by the DNS lookup, all the IP addresses are included in $dnslist_value, separated by commas and spaces.

You can use these variables in message or log_message modifiers – although these appear before the condition in the ACL, they are not expanded until after it has failed. For example:

deny    hosts = !+local_networks
        message = $sender_host_address is listed \
                  at $dnslist_domain
        dnslists = rbl-plus.mail-abuse.example

You can add an equals sign and an IP address after a dnslists domain name in order to restrict its action to DNS records with a matching right hand side. For example,

deny dnslists = rblplus.mail-abuse.org=127.0.0.2

rejects only those hosts that yield 127.0.0.2. Without this additional data, any address record is considered to be a match. If more than one address record is found on the list, they are all checked for a matching right-hand side.

More than one IP address may be given for checking, using a comma as a separator. These are alternatives – if any one of them matches, the dnslists condition is true. For example:

deny  dnslists = a.b.c=127.0.0.2,127.0.0.3

If you want to specify a constraining address list and also specify names or IP addresses to be looked up, the constraining address list must be specified first. For example:

deny dnslists = dsn.rfc-ignorant.org\
                =127.0.0.2/$sender_address_domain

If the character “&” is used instead of “=”, the comparison for each listed IP address is done by a bitwise “and” instead of by an equality test. In other words, the listed addresses are used as bit masks. The comparison is true if all the bits in the mask are present in the address that is being tested. For example:

dnslists = a.b.c&0.0.0.3

matches if the address is 'x.x.x.'3, 'x.x.x.'7, 'x.x.x.'11, etc. If you want to test whether one bit or another bit is present (as opposed to both being present), you must use multiple values. For example:

dnslists = a.b.c&0.0.0.1,0.0.0.2

matches if the final component of the address is an odd number or two times an odd number.

You can supply a negative list of IP addresses as part of a dnslists condition. Whereas

deny  dnslists = a.b.c=127.0.0.2,127.0.0.3

means “deny if the host is in the black list at the domain a.b.c and the IP address yielded by the list is either 127.0.0.2 or 127.0.0.3”,

deny  dnslists = a.b.c!=127.0.0.2,127.0.0.3

means “deny if the host is in the black list at the domain a.b.c and the IP address yielded by the list is not 127.0.0.2 and not 127.0.0.3”. In other words, the result of the test is inverted if an exclamation mark appears before the “=” (or the “&”) sign.

Note: this kind of negation is not the same as negation in a domain, host, or address list (which is why the syntax is different).

If you are using just one list, the negation syntax does not gain you much. The previous example is precisely equivalent to

deny  dnslists = a.b.c
     !dnslists = a.b.c=127.0.0.2,127.0.0.3

However, if you are using multiple lists, the negation syntax is clearer. Consider this example:

deny  dnslists = sbl.spamhaus.org : \
                 list.dsbl.org : \
                 dnsbl.njabl.org!=127.0.0.3 : \
                 relays.ordb.org

Using only positive lists, this would have to be:

deny  dnslists = sbl.spamhaus.org : \
                 list.dsbl.org
deny  dnslists = dnsbl.njabl.org
     !dnslists = dnsbl.njabl.org=127.0.0.3
deny  dnslists = relays.ordb.org

which is less clear, and harder to maintain.

If Exim is asked to do a dnslist lookup for an IPv6 address, it inverts it nibble by nibble. For example, if the calling host’s IP address is 3ffe:ffff:836f:0a00:000a:0800:200a:c031, Exim might look up

1.3.0.c.a.0.0.2.0.0.8.0.a.0.0.0.0.0.a.0.f.6.3.8.
  f.f.f.f.e.f.f.3.blackholes.mail-abuse.org

(split over two lines here to fit on the page). Unfortunately, some of the DNS lists contain wildcard records, intended for IPv4, that interact badly with IPv6. For example, the DNS entry

*.3.some.list.example.    A    127.0.0.1

is probably intended to put the entire 3.0.0.0/8 IPv4 network on the list. Unfortunately, it also matches the entire 3::/4 IPv6 network.

You can exclude IPv6 addresses from DNS lookups by making use of a suitable condition condition, as in this example:

deny   condition = ${if isip4{$sender_host_address}}
       dnslists  = some.list.example

ln(peakrate/maxrate)

# Log all senders' rates
warn
  ratelimit = 0 / 1h / strict
  log_message = Sender rate $sender_rate / $sender_rate_period

# Slow down fast senders
warn
  ratelimit = 100 / 1h / per_rcpt / strict
  delay     = ${eval: $sender_rate - $sender_rate_limit }s

# Keep authenticated users under control
deny
  ratelimit = 100 / 1d / strict / $authenticated_id

# System-wide rate limit
defer
  message = Sorry, too busy. Try again later.
  ratelimit = 10 / 1s / $primary_hostname

# Restrict incoming rate from each host, with a default
# set using a macro and special cases looked up in a table.
defer
  message = Sender rate exceeds $sender_rate_limit \
            messages per $sender_rate_period
  ratelimit = ${lookup {$sender_host_address} \
                cdb {DB/ratelimits.cdb} \
                {$value} {RATELIMIT} }

Several of the verify conditions described in section 39.20 cause addresses to be verified. These conditions can be followed by options that modify the verification process. The options are separated from the keyword and from each other by slashes, and some of them contain parameters. For example:

verify = sender/callout
verify = recipient/defer_ok/callout=10s,defer_ok

The first stage of address verification, which always happens, is to run the address through the routers, in “verify mode”. Routers can detect the difference between verification and routing for delivery, and their actions can be varied by a number of generic options such as verify and verify_only (see chapter 15). If routing fails, verification fails. The available options are as follows:

warn  !verify = sender
       set acl_m0 = $acl_verify_message

In addition, $sender_verify_failure or $recipient_verify_failure (as appropriate) contains one of the following words:

The main use of these variables is expected to be to distinguish between rejections of MAIL and rejections of RCPT in callouts.

Recipient callouts are usually used only between hosts that are controlled by the same administration. For example, a corporate gateway host could use callouts to check for valid recipients on an internal mailserver. A successful callout does not guarantee that a real delivery to the address would succeed; on the other hand, a failing callout does guarantee that a delivery would fail.

If the callout option is present on a condition that verifies an address, a second stage of verification occurs if the address is successfully routed to one or more remote hosts. The usual case is routing by a dnslookup or a manualroute router, where the router specifies the hosts. However, if a router that does not set up hosts routes to an smtp transport with a hosts setting, the transport’s hosts are used. If an smtp transport has hosts_override set, its hosts are always used, whether or not the router supplies a host list.

The port that is used is taken from the transport, if it is specified and is a remote transport. (For routers that do verification only, no transport need be specified.) Otherwise, the default SMTP port is used. If a remote transport specifies an outgoing interface, this is used; otherwise the interface is not specified.

For a sender callout check, Exim makes SMTP connections to the remote hosts, to test whether a bounce message could be delivered to the sender address. The following SMTP commands are sent:

LHLO is used instead of HELO if the transport’s protocol option is set to “lmtp”.

A recipient callout check is similar. By default, it also uses an empty address for the sender. This default is chosen because most hosts do not make use of the sender address when verifying a recipient. Using the same address means that a single cache entry can be used for each recipient. Some sites, however, do make use of the sender address when verifying. These are catered for by the use_sender and use_postmaster options, described in the next section.

If the response to the RCPT command is a 2’xx’ code, the verification succeeds. If it is 5xx, the verification fails. For any other condition, Exim tries the next host, if any. If there is a problem with all the remote hosts, the ACL yields “defer”, unless the defer_ok parameter of the callout option is given, in which case the condition is forced to succeed.

The callout option can be followed by an equals sign and a number of optional parameters, separated by commas. For example:

verify = recipient/callout=10s,defer_ok

The old syntax, which had callout_defer_ok and check_postmaster as separate verify options, is retained for backwards compatibility, but is now deprecated. The additional parameters for callout are as follows:

If you use any of the parameters that set a non-empty sender for the MAIL command (mailfrom, postmaster_mailfrom, use_postmaster, or use_sender), you should think about possible loops. Recipient checking is usually done between two hosts that are under the same management, and the host that receives the callouts is not normally configured to do callouts itself. Therefore, it is normally safe to use use_postmaster or use_sender in these circumstances.

However, if you use a non-empty sender address for a callout to an arbitrary host, there is the likelihood that the remote host will itself initiate a callout check back to your host. As it is checking what appears to be a message sender, it is likely to use an empty address in MAIL, thus avoiding a callout loop. However, to be on the safe side it would be best to set up your own ACLs so that they do not do sender verification checks when the recipient is the address you use for header sender or postmaster callout checking.

Another issue to think about when using non-empty senders for callouts is caching. When you set mailfrom or use_sender, the cache record is keyed by the sender/recipient combination; thus, for any given recipient, many more actual callouts are performed than when an empty sender or postmaster is used.

Exim caches the results of callouts in order to reduce the amount of resources used, unless you specify the no_cache parameter with the callout option. A hints database called “callout” is used for the cache. Two different record types are used: one records the result of a callout check for a specific address, and the other records information that applies to the entire domain (for example, that it accepts the local part postmaster).

When an original callout fails, a detailed SMTP error message is given about the failure. However, for subsequent failures use the cache data, this message is not available.

The expiry times for negative and positive address cache records are independent, and can be set by the global options callout_negative_expire (default 2h) and callout_positive_expire (default 24h), respectively.

If a host gives a negative response to an SMTP connection, or rejects any commands up to and including

MAIL FROM:<>

(but not including the MAIL command with a non-empty address), any callout attempt is bound to fail. Exim remembers such failures in a domain cache record, which it uses to fail callouts for the domain without making new connections, until the domain record times out. There are two separate expiry times for domain cache records: callout_domain_negative_expire (default 3h) and callout_domain_positive_expire (default 7d).

Domain records expire when the negative expiry time is reached if callouts cannot be made for the domain, or if the postmaster check failed. Otherwise, they expire when the positive expiry time is reached. This ensures that, for example, a host that stops accepting “random” local parts will eventually be noticed.

The callout caching mechanism is based on the domain of the address that is being tested. If the domain routes to several hosts, it is assumed that their behaviour will be the same.

When sender verification fails in an ACL, the details of the failure are given as additional output lines before the 550 response to the relevant SMTP command (RCPT or DATA). For example, if sender callout is in use, you might see:

MAIL FROM:<xyz@abc.example>
250 OK
RCPT TO:<pqr@def.example>
550-Verification failed for <xyz@abc.example>
550-Called:   192.168.34.43
550-Sent:     RCPT TO:<xyz@abc.example>
550-Response: 550 Unknown local part xyz in <xyz@abc.example>
550 Sender verification failed

If more than one RCPT command fails in the same way, the details are given only for the first of them. However, some administrators do not want to send out this much information. You can suppress the details by adding “/no_details” to the ACL statement that requests sender verification. For example:

verify = sender/no_details

A dilemma arises when a local address is redirected by aliasing or forwarding during verification: should the generated addresses themselves be verified, or should the successful expansion of the original address be enough to verify it? By default, Exim takes the following pragmatic approach:

This seems the most reasonable behaviour for the common use of aliasing as a way of redirecting different local parts to the same mailbox. It means, for example, that a pair of alias entries of the form

A.Wol:   aw123
aw123:   :fail: Gone away, no forwarding address

work as expected, with both local parts causing verification failure. When a redirection generates more than one address, the behaviour is more like a mailing list, where the existence of the alias itself is sufficient for verification to succeed.

require verify = recipient/success_on_redirect/callout=10s

verify = csa

verify = csa/$sender_helo_name

verify = csa/$sender_host_address

${lookup dnsdb {csa=$sender_helo_name}}

An MTA is said to relay a message if it receives it from some host and delivers it directly to another host as a result of a remote address contained within it. Redirecting a local address via an alias or forward file and then passing the message on to another host is not relaying, but a redirection as a result of the “percent hack” is.

Two kinds of relaying exist, which are termed “incoming” and “outgoing”. A host which is acting as a gateway or an MX backup is concerned with incoming relaying from arbitrary hosts to a specific set of domains. On the other hand, a host which is acting as a smart host for a number of clients is concerned with outgoing relaying from those clients to the Internet at large. Often the same host is fulfilling both functions,

but in principle these two kinds of relaying are entirely independent. What is not wanted is the transmission of mail from arbitrary remote hosts through your system to arbitrary domains.

You can implement relay control by means of suitable statements in the ACL that runs for each RCPT command. For convenience, it is often easiest to use Exim’s named list facility to define the domains and hosts involved. For example, suppose you want to do the following:

In the main part of the configuration, you put the following definitions:

domainlist local_domains = my.dom1.example : my.dom2.example
domainlist relay_domains = friend1.example : friend2.example
hostlist   relay_hosts   = 192.168.45.0/24

Now you can use these definitions in the ACL that is run for every RCPT command:

acl_check_rcpt:
  accept domains = +local_domains : +relay_domains
  accept hosts   = +relay_hosts

The first statement accepts any RCPT command that contains an address in the local or relay domains. For any other domain, control passes to the second statement, which accepts the command only if it comes from one of the relay hosts. In practice, you will probably want to make your ACL more sophisticated than this, for example, by including sender and recipient verification. The default configuration includes a more comprehensive example, which is described in chapter 7.

You can check the relay characteristics of your configuration in the same way that you can test any ACL behaviour for an incoming SMTP connection, by using the -bh option to run a fake SMTP session with which you interact.

For specifically testing for unwanted relaying, the host relay-test.mail-abuse.org provides a useful service. If you telnet to this host from the host on which Exim is running, using the normal telnet port, you will see a normal telnet connection message and then quite a long delay. Be patient. The remote host is making an SMTP connection back to your host, and trying a number of common probes to test for open relay vulnerability. The results of the tests will eventually appear on your terminal.