http://wiki.zimbra.com/wiki/Configuring_maxmessagesize

Check the current settings:

su - zimbra
zmprov gcf zimbraFileUploadMaxSize
zmprov gcf zimbraMtaMaxMessageSize
zmprov gcf zimbraMailContentMaxSize

This sets a 40MB maximum message size (after MIME encoding):

su - zimbra
zmprov mcf zimbraFileUploadMaxSize 40960000
zmprov mcf zimbraMtaMaxMessageSize 40960000
zmprov mcf zimbraMailContentMaxSize 81920000
postfix reload
zmcontrol restart

If you still have issues with maximum message size, reboot the server before troubleshooting further.

Is it necessary to reload the Postfix config if you restart Zimbra?