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 use Milter (a Python based extension for Postfix) to add/replace some headers to make Zimbra distribution lists act as mailing lists. In addition it is showed how you can enforce posting restrictions.

The following headers will be added/replaced:

Reply-To: [email protected]
Precedence: list
List-Id: [email protected]
List-Post: mailto:[email protected]
Errors-To: [email protected]
Sender: [email protected]

On the CLI:

 yum install epel-release
 yum install python-pip supervisor
 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.

If you are satisfied, you can 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
 #don't need this 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