Configuring maxmessagesize
The following explains how to observe and adjust the Maximum Message Size for messages passing through the Zimbra MTA (Postfix). The message_size_limit postconf parameter is configured globally for all Zimbra MTAs. The default Zimbra MTA configuration uses the default Postfix message_size_limit of 10MB (i.e. '10240000' bytes). Note that this is the size of the full RFC 2822 internet message, after any necessary MIME-encoding.
Postfix configuration
You can examine the current value of this parameter like this:
# su - zimbra $ postconf message_size_limit message_size_limit = 10240000
This configuration parameter is stored in the zimbra ldap directory, and propagated to postconf's message_size_limit by zmmtaconfig, which is invoked by the zimbra postfix command. The following commands will set the message_size_limit to 2MB (adjust this value to suit your needs):
# su - zimbra $ zmprov modifyConfig zimbraMtaMaxMessageSize 2048000 $ postfix reload
You can then confirm the changes with this command:
$ postconf | grep message_size_limit
Reference:
http://www.postfix.org/resource.html