NAME

       verifyfilter, verifysmtp - Verify mail addresses

SYNOPSIS

       filterctl {[start] | [stop]} verifyfilter

DESCRIPTION

       This is a mail filter/tool combination that tries to determine the validity of E-mail addresses by
       attempting to contact the mail domain's mail server, and executing a RCPT TO command. There are three
       ways to use this tool.

       •   As a shell command, to test an E-mail address for deliverability.

       •   As a global mail filter.

       •   As a local recipient mail filter.

       verifyfilter is suitable for handling moderate amount of E-mail traffic. Address validation is expensive,
       requiring a DNS lookup and an outbound connection to a mail server for every validated address.
       verifyfilter goes through the same steps that the mail server does when sending mail, including enabling
       of encryption.

       verifyfilter uses logging and caching, to avoid repeatedly validating the same return address when
       receiving multiple E-mails from the same sender; but its caching is rudimentary (a simple log file), and
       increased logging due to high E-mail traffic mail delivery may create large log files, impacting
       performance.

   The verifysmtp command

       verifysmtp [-n] [-t directory] [-m {full|base}] {user@domain}

       The verifysmtp command creates a network connection to domain's mail server, and checks if it considers
       the given E-mail address as valid. Two or more E-mail addresses can be given, and each E-mail address
       gets checked individually.

       verifysmtp terminates with a zero exit code if all given E-mail addresses passed. A non-zero exit code
       indicates that one or more of the given addresses were rejected.

       The -m option is analogous to the verifyfilter-logmode setting, described below, that specifies how
       E-mail addresses are compared against the cached verification results. The -t enables caching of
       verification results, and specifies the directory for storing the cached results.

       The -n option suppresses internal error messages from getting logged to standard error. This is used in
       the verifyfilter global mail filter.

   The verifyfilter global mail filter

       filterctl {[start] | [stop]} verifyfilter

       The verifyfilter global mail filter, if enabled, checks each message's return address. The E-mail message
       gets rejected if its return address's mail server rejects the return address. There's not much sense in
       accepting mail if its return address is undeliverable.

       verifyfilter ignores messages from authenticated senders, and does not check their return addresses.

   Local recipient mail filter
           mkdir /etc/courier/maildroprcs
           cp /usr/lib/courier/verifysender /etc/courier/maildroprcs
           cp /usr/lib/courier/verifysenderfull /etc/courier/maildroprcs

       In your $HOME:

           mkdir $HOME/.trackdir

       In $HOME/.mailfilters/rcptfilter:

           include '/etc/courier/maildroprcs/verifysender'

       Or:

           include '/etc/courier/maildroprcs/verifysenderfull'

       This alternative provides comparable functionality as the global mail filter, but exposed via the
       localmailfilter(7)[1] API.

       With maildrop, a protected wrapper filtering recipe gets installed into
       /etc/courier/maildroprcs/verifysender, which invokes verifysmtp via maildrop's system command. The
       wrapper must be included in this manner, since maildrop normally does not allow the system command in the
       embedded execution mode that's used by the local mail filtering API.

       The wrapper executes verifysmtp with a special argument, a single “.”. This is a special parameter that
       indicates that verifysmtp should read the E-mail address from the SENDER environment variable (avoiding
       issues with shell expansion, and script kiddies).

       The /etc/courier/maildroprcs/verifysender wrapper can be suitably, perhaps optionally, included from
       either the rcptfilter or the smtpfilter script.

   Logging and caching
       verifyfilter caches the return address it checks. Once verifyfilter verifies that the return address is
       accepted by the sending domain's mail server, this is logged and additional E-mail with the same return
       address gets immediately accepted without contacting the sending domain's mail server to re-check the
       same address.

       Successfully verified return addresses get cached for approximately 2-3 hours. If no other E-mail with
       the same address get received before the cache expires that return address gets rechecked the next time
       it is seen. If another E-mail with the same return address gets received, it is immediately accepted and
       the email address gets recached. Most mailing lists' bounce addresses should not cause excessive
       re-verification, provided a regular trickle of mailing list traffic. This includes mailing list that use
       per-message bounce addresses that follow the common VERP convention (Variable Envelope Return Path), see
       “FILES” below.

       As becomes obvious after perusing the contents of the stock verifysender local mail filter, verifysmtp's
       -t gives the name of a scratch directory that verifysmtp uses for the cache. This directory should be set
       aside for verifysmtp, and not used for other purposes. The global mail filter shares the scratch
       directory together with other Courier functions that work the same way.

BUGS

       Just because a mail server accepted the RCPT TO it does not mean that the given E-mail address can
       ultimately receive E-mail. This is especially true if a smarthost is in the picture.  verifyfilter and
       verifysmtp will attempt to read the esmtproutes configuration file (see courier(8)[2] for more
       information). If esmtproutes is configured to use a smarthost all mail, for all domains, then every email
       address will pass validation if the smarthost accept mail to any recipient without any further checking
       (as is usually the case).

       verifysmtp is a shell script wrapper that imports Courier's environment from /etc/courier/courierd then
       executes verifyfilter. Running as a non-daemon user, verifysmtp may not be able to read some Courier's
       configuration files, like the aforementioned esmtproutes and authclient. As such, verifysmtp's behavior
       might differ from Courier's, when sending mail.

FILES

       verifyfilter uses the following configuration files. Changes to the following files do not take effect
       until the filter has been stopped and restarted.

       /etc/courier/filters/verifyfilter-mode
           If this file exists and contains the word "all", verifyfilter will create its socket in
           /var/lib/courier/allfilters, otherwise the socket will be created in /var/lib/courier/filters, see
           courierfilter(8)[3] for more information.

       /etc/courier/filters/verifyfilter-logmode
           This file contains a single line, “base” or “full”.  “full” verifies each return address, exactly as
           given. The default “base” value strips off any VERP component of the sender's address, before
           comparing it. If the local mailbox portion of the return address contains a dash, the dash and the
           remaining portion of the return address gets removed.

           With the default “base” logging mode, once the E-mail address <list-12@example.com> gets verified,
           all further addresses like <list-13@example.com> and <list-foo-bar@example.com> are considered as
           verified, but not <anotherlist@example.com>.

       /etc/courier/filters/verifyfilter-nthreads
           This file contains a single numerical value that sets the number of threads created (one thread is
           used to check each email address). The default number of threads is 10.

SEE ALSO

       courierfilter(8)[3], localmailfilter(7)[1].

AUTHOR

       Sam Varshavchik
           Author

NOTES

        1.

                  localmailfilter(7)
           https://www.courier-mta.org/localmailfilter.html

        2.

                courier(8)

           https://www.courier-mta.org/courier.html

        3. courierfilter(8)
           https://www.courier-mta.org/courierfilter.html

Courier Mail Server                                12/04/2024                                    VERIFYFILTER(8)