I've noticed that if I check "Enable Milter Server" under the MTA tab in Server settings, com.zimbra.cs.milter.MinaMilterServer is launched via zmmilterctl and listens on port 7026. Googling it has not been informative.

I've been able to add a different milter listening on a different port using the zimbraMtaSmtpdMilters variable but I'm still curious what com.zimbra.cs.milter.MinaMilterServer does exactly and how to configure it, if it's even meant to be configurable.