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 RCPTTO 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.
Theverifysmtpcommandverifysmtp [-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.
Theverifyfilterglobalmailfilterfilterctl {[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.
Localrecipientmailfilter
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.
Loggingandcachingverifyfilter 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.
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