Set mailing list email headers with Milter

This wiki page shows how to work around the problem of mail going to junk/spam folder for external recipients of Zimbra distribution lists.

By default Zimbra distribution lists do not set appropriate e-mail headers. This wiki page shows how to set-up Milter (a Python based extension for Postfix). Basically what Milter is set-up to do is filter the original message and discard it. Before discarding, it takes some headers and the body from the original mail and place them in a new mail and use sendmail to pass it to the DL.

The result is a clean message from the correct domain, and it should be able to pass SPF/DKIM and all that.

Its great, because the final message that goes to the recipients is still processed by Zimbra as a normal DL message.

On the CLI:

 yum install epel-release
 yum install python-pip supervisor python-pymilter sendmail
 pip install --upgrade pip     
 pip install python-libmilter

 mkdir - p /opt/zimbra_mailinglists_milter
 cd /opt/zimbra_mailinglists_milter
 wget https://raw.githubusercontent.com/Zimbra-Community/mailing-lists/master/milter/zimbra_mailinglists_milter.py -O /opt/zimbra_mailinglists_milter/zimbra_mailinglists_milter.py
 wget https://raw.githubusercontent.com/Zimbra-Community/mailing-lists/master/milter/etc/daemon.ini -O /etc/supervisord.d/zimbra_mailinglists_milter.ini

Then use your favorite editor (nano/vim) and open /opt/zimbra_mailinglists_milter/zimbra_mailinglists_milter.py look under def eob(self , cmdDict) and change the script to fit your needs. Please be aware that the indentation level of the statements is significant to Python.

While Zimbra comes with a built-in sendmail, it's configuration cannot be altered, so we use sendmail from the OS/distro. Configure it by setting 127.0.0.1 after DS to set Zimbra as the Smart Relay for the OS/distro:

 nano /etc/mail/sendmail.cf
 # "Smart" relay host (may be null)
 DS127.0.0.1

Enable and start the service.

 chmod +rx /opt/zimbra_mailinglists_milter/zimbra_mailinglists_milter.py
 systemctl start supervisord 
 systemctl enable supervisord
 tail -f /var/log/supervisor/supervisord.log
 netstat -tulpn | grep 5000 #should show the service

If you made any typos, you will see them in the log, and there will be nothing listening on port 5000. Try again and issue systemctl stop supervisord && systemctl start supervisord.

If it works, enable it for Zimbra:

 su - zimbra
 zmprov ms `zmhostname` zimbraMtaSmtpdMilters inet:127.0.0.1:5000
 zmprov ms `zmhostname` zimbraMtaNonSmtpdMilters inet:127.0.0.1:5000
 zmprov ms `zmhostname` zimbraMilterBindPort 5000
 zmprov ms `zmhostname` zimbraMilterServerEnabled TRUE
 zmmtactl reload

Try sending some emails and:

 tail -f /var/log/zimbra_mailinglists_milter.log

In case Zimbra says the MTA is not running:

De above settings make Zimbra think it has the built-in Milter running as well, when in fact it is not. At this time I do not know the proper way to configure this. One work-around is, commenting out a section in zmmilterctl:

 nano /opt/zimbra/bin/zmmilterctl
   status)
     echo -n "Milter server is "
 #    checkrunning
 #    if [ $running = 0 ]; then
 #      echo "not running."
 #      exit 1
 #    else
       echo "running."
       exit 0
 #    fi