The following provides instructions how to enable a Maximum Message Size for messages passing through the Zimbra MTA (ie. Postfix). The "maxmessagesize" config is unique to each Zimbra MTA you have (ie. if you have a distributed Zimbra deployment, you'll need to make change in each MTA host where you want this maxmessagesize limit enforced).
The following commands will set the message_size_limit to 2MB, you can adjust this value to your needs:
su - zimbra postconf -e message_size_limit='' ; this will clear the current value zmprov mcf zimbraMtaMaxMessageSize 20000 wait two minutes for the changes to be published then confirm the changes with this command postconf | grep message_size_limit