Integrating SpamAssassin into Postfix using spamd

To integrate SpamAssassin into Postfix, you'll need to pipe incoming e-mail through a script or program that passes the e-mail to SpamAssassin for rewriting, and then either chooses to send it on or discard it (you may wish to discard it, for example, if SpamAssassin reports a very high spam score).

Below are some examples of how to do this using your own script to feed e-mail to SpamAssassin. Aside from the methods below, you can look in the examples folder included with SpamAssassin (located, for example, at /usr/share/doc/spamassassin/examples on Debian installs by default) for other options, like procmail.

Note that it is generally good practice to configure Postfix NOT to bounce undeliverable messages, as this can cause backscatter. Either have a 'catch-all' address that all e-mail to unknown senders goes to, or just silently discard e-mail to unknown senders. Never bounce e-mail detected as spam; either rewrite it and deliver it, or silently discard it.

bash script to rewrite spam

This is a simple method to integrate SpamAssassin. All spam e-mail will still come through to your address(es), but it will be clearly marked as spam.

1) Create a bash script to receive e-mail from Postfix and pipe it to SpamAssassin for rewriting. Then forward the rewritten version to Postfix's sendmail implementation:

#!/bin/sh
# 
# This script should probably live at /usr/bin/spamfilter.sh
# ... and have 'chown root:root' and 'chmod 755' applied to it.
# 
# Simple filter to plug SpamAssassin into the Postfix MTA
# 
# Modified by Jeremy Morton
# 
# For use with:
#    Postfix 20010228 or later
#    SpamAssassin 2.42 or later
# 
# Note: Modify the file locations to match your particular
#       server and installation of SpamAssassin.

# File locations:
# (CHANGE AS REQUIRED TO MATCH YOUR SERVER)
SENDMAIL=/usr/sbin/sendmail
SPAMASSASSIN=/usr/bin/spamc

logger "Spam filter piping to SpamAssassin, then to: $SENDMAIL $@"

/bin/cat | ${SPAMASSASSIN} | ${SENDMAIL} "$@"

exit $?

This, of course, assumes you're using the spamd/spamc implementation of SpamAssassin to improve performance - spamc is the command you'll want to invoke to check an e-mail (by setting SPAMASSASSIN to its location).

2) Ensure the newly-created /usr/bin/spamfilter.sh has correct permissions (0755), and is owned by root:root.

3) Modify the /etc/postfix/master.cf file; first, change the first 'smtp' line of the file to:

  smtp      inet  n       -       -       -       -       smtpd -o content_filter=spamfilter

Then, add the following (a call to our newly-created spamfilter script) at the end:

  spamfilter
            unix  -       n       n       -       -       pipe
     flags=Rq user=spamd argv=/usr/bin/spamfilter.sh -oi -f ${sender} ${recipient}

This setup assumes you have a 'spamd' user for the script to be run as. If you wish to run it as a different user, modify the 'user' argument above.

4) Restart the Postfix service.

Now, Postfix should be sending every incoming e-mail through the spamfilter.sh script (which means it's going through SpamAssassin), with the e-mail headers (and maybe the rest of the e-mail, if it's spam) being rewritten before being delivered to the target mailbox. Optionally, you may wish to tweak SpamAssassin's configuration to cause it to rewrite the e-mails in a more useful way (only rewrite spam mail subject but not whole e-mail, add a 'ham' header to give detailed info on even non-spam, etc.)

5) (optional) Tweak SpamAssasin's configuration. Ensure that the below is in /etc/mail/spamassassin/local.cf:

  # Rewrite the subject of suspected spam e-mails
  rewrite_header Subject *****SPAM*****
  # Just add an X-Spam-Report header to suspected spam, rather than rewriting the content of the e-mail
  report_safe 0
  # Also we want to add a detailed ham report header to even e-mail that ISN'T suspected to be spam
  add_header ham HAM-Report _REPORT_
  # Set the threshold at which a message is considered spam (3 is usually sufficient)
  required_score 3.0

Perl script to discard very obvious spam

TODO: describe a similar method to above, but with a Perl script that looks at the rewritten headers and discards spam that is, say, scored higher than 7. That means < 3 is OK, 3-7 is marked as spam but delivered, > 7 is discarded.

Old alternative

A major problem with the old method filter (below) is that Postfix attempts to bounce messages flagged as spam. This is a waste of resources as most spams have fake return addresses. Furthermore, if they fake a return address that doesn't belong to them, you may end up bouncing the message to an innocent 3rd party. Enough of this can result in YOU being on an RBL.

_ Not true. (1) Postfix only bounces messages if you tell it to. I currently use the above filter, and I use a sieve rule to filter X-Spam-Flag: YES into a folder in the same account which I occasionally review. Many people sort mailing lists into separate folders, and this can typically be done with the same mechanism. It also makes it more practical to actually check it every now and then... all spam systems have false positives, so silently blackholing likely spam (potential ham) is far more rude than bouncing it. (2) If you use a proper bounce message (as opposed to those stupid virus filter notifications), there are ways for the innocent party to filter out bounces from messages they didn't send. When I get some time, I'll write one up and link to it. -Slamb_

Instead, we'd like to create a blackhole for spam.

To accomplish this while still using an after-queue filter, the spam may be redirected to another account on the system. Steps to do this are as follows:

1) Modify /etc/postfix/master.cf with:

smtp      inet  n       -       n       -       -       smtpd -o content_filter=spamassassin

... and then at the end, add:

spamassassin
          unix  -       n       n       -       -       pipe
   flags=Rq user=nobody argv=/path/to/filter.sh -oi -f ${sender} ${recipient}

2) Create filter.sh as follows:

#!/bin/sh

# filter.sh
#
# This script redirects mail flagged as spam to a separate account
# You must first create a user account named "spamvac" to hold the flagged mail

SENDMAIL="/usr/sbin/sendmail -i"
SPAMASSASSIN=/usr/bin/spamc
COMMAND="$SENDMAIL $@"
#If your SQL preferences set to "user"
USER=`echo $COMMAND | awk '{ print $NF }' | sed 's/@.*$//'`
#If your SQL preferences set to "user@domain"
#USER=`echo $COMMAND | awk '{ print $NF }'`

NEW_COMMAND=`echo $COMMAND | awk '{ $6 = "spamvac"; NF = 6; print }'`

# Exit codes from <sysexits.h>
EX_TEMPFAIL=75
EX_UNAVAILABLE=69

umask 077

OUTPUT="`mktemp /tmp/mailfilter.XXXXXXXXXX`"

if [ "$?" != 0 ]; then
    /usr/bin/logger -s -p mail.warning -t filter "Unable to create temporary file."
    exit $EX_TEMPFAIL
fi

# Clean up when done or when aborting.
trap "rm -f $OUTPUT" EXIT TERM

$SPAMASSASSIN -x -E -u $USER > $OUTPUT
return="$?"
if [ "$return" = 1 ]; then
    $NEW_COMMAND < $OUTPUT
    exit $?
elif [ "$return" != 0 ]; then
    /usr/bin/logger -s -p mail.warning -t filter "Temporary SpamAssassin failure (spamc returned $return)"
    exit $EX_TEMPFAIL
fi

$SENDMAIL "$@" < $OUTPUT
exit $?

This causes incoming smtp mail to be checked for spam. If the mail's spam score exceeds the threshold (set in local.cf, or in ~/.spamassassin/user_prefs) then spamc -E returns 1 and the mail is redirected to the spamvac account. Otherwise it is passed on to the intended recipient. Bounces no longer occur except on a true error condition.

Old method (original)

The easiest way to integrate postfix and spamassassin is to use spamd in an after-queue inspection. This configuration does not allow rejecting messages within the SMTP transaction, so it unfortunately contributes to backscatter email. On the other hand, it has important performance advantages over before-queue inspection.

First, edit /etc/postfix/master.cf, find the

# ==========================================================================
# service type  private unpriv  chroot  wakeup  maxproc command + args
#               (yes)   (yes)   (yes)   (never) (50)
# ==========================================================================
...
smtp      inet  n       -       n       -       -       smtpd
...

line and just add " -o content_filter=spamassassin" to the end of the line:

# ==========================================================================
# service type  private unpriv  chroot  wakeup  maxproc command + args
#               (yes)   (yes)   (yes)   (never) (50)
# ==========================================================================
...
smtp      inet  n       -       n       -       -       smtpd -o content_filter=spamassassin
...

Then, go to the end of the file, and add this:

# ==========================================================================
# service type  private unpriv  chroot  wakeup  maxproc command + args
#               (yes)   (yes)   (yes)   (never) (50)
# ==========================================================================
...
spamassassin
          unix  -       n       n       -       -       pipe
   user=nobody argv=/path/to/spamc -e /path/to/postfix/sendmail -oi -f ${sender} ${recipient}
# make sure it's all on one line or
# start the consecutive lines with a whitespace
# like I did here

Make sure that you have adjusted the path to the spamc and sendmail commands above! (Please note that the path required is Postfix's sendmail and not the standalone package, Sendmail the SMTP server. It will not work if you're not careful about which one is installed). Then, setup spamd to start with the system, and you are ready to go. If you wish to provide spamassassin preferences, change "user=nobody" to a valid system user (except for root, since Postfix will NOT call external programs as root), and add .spamassassin into that user's home directory.

If you want spamd to setuid to the user so it can save settings and leaning data you need to tell it the user

# ==========================================================================
# service type  private unpriv  chroot  wakeup  maxproc command + args
#               (yes)   (yes)   (yes)   (never) (50)
# ==========================================================================
...
spamassassin
          unix  -       n       n       -       -       pipe
   user=nobody argv=/path/to/spamc -u ${user} -e /path/to/postfix/sendmail -oi -f ${sender} ${recipient}

If you use user preferences stored in SQL, you should change "spamassassin" service in master.cf to following:

# ==========================================================================
# service type  private unpriv  chroot  wakeup  maxproc command + args
#               (yes)   (yes)   (yes)   (never) (50)
# ==========================================================================
...
spamassassin
          unix  -       n       n       -       -       pipe
   flags=Rq user=nobody argv=/path/to/spamc -u ${user} -e /path/to/postfix/sendmail -oi -f ${sender} ${recipient}

Notice "-u ${recipient}" added. Otherwise "username" field in database will always appear as user which postfix is invoking spamc(in this example it is 'nobody'). Do not forget also to set spamassassin_destination_recipient_limit = 1 in main.cf . spamc doesn't expect to parse multiple recipients at once.

Note that this exact method of invoking SpamAssassin is not very robust. Postfix's pipe error behavior is as follows:

If the exec() fails (e.g., /path/to/spamc is incorrect), Postfix will bounce the message.
If the command returns EX_OSERR or EX_TEMPFAIL (defined in /usr/include/sysexits.h), Postfix will defer the message.
If the command returns EX_OK, Postfix will consider the message done. The command should deliver it; it will be discarded otherwise.
If the command returns any other error code, Postfix will bounce the message.

With spamc -e, spamc will return EX_OK when spamd is unavailable but sendmail is working, so Postfix will deliver the message. You probably want it to be deferred instead. This is not possible with spamc directly, but you can do so with a simple script. Replace the lines above with:

spamassassin
          unix  -       n       n       -       -       pipe
   flags=Rq user=nobody argv=/path/to/filter.sh -oi -f ${sender} ${recipient}

and create a filter.sh as follows:

#!/bin/bash
SENDMAIL="/usr/sbin/sendmail -i"
SPAMASSASSIN=/usr/bin/spamc

# Exit codes from <sysexits.h>
EX_TEMPFAIL=75
EX_UNAVAILABLE=69

umask 077

OUTPUT="`mktemp -p /tmp mailfilter.XXXXXXXXXX`"
if [ "$?" != 0 ]; then
    /usr/bin/logger -s -p mail.warning -t filter \
        "Unable to create temporary file."
    exit $EX_TEMPFAIL
fi

# Clean up when done or when aborting.
trap "rm -f $OUTPUT" EXIT TERM

$SPAMASSASSIN -x > $OUTPUT
return="$?"
if [ "$return" = 1 ]; then
    echo "Message content rejected"
    exit $EX_UNAVAILABLE
elif [ "$return" != 0 ]; then
    /usr/bin/logger -s -p mail.warning -t filter \
        "Temporary SpamAssassin failure (spamc returned $return)"
    exit $EX_TEMPFAIL
fi

$SENDMAIL "$@" < $OUTPUT
exit $?

You still must be careful that the script is in the right place and executable or email will bounce. However, this will defer messages on most failures.

A more complex solution is the one proposed in the FILTER_README file that comes with the Postfix distribution. See IntegratePostfixViaSpampd for one approach.

Only mail received by SMTP will be scanned with this method, i.e. mail injected with sendmail(1) will not be fed to SpamAssassin.

Child pages

IntegratedSpamdInPostfix

Integrating SpamAssassin into Postfix using spamd

bash script to rewrite spam

Perl script to discard very obvious spam

Old alternative

Old method (original)