Hi
Welcome to the forums.
Do you actually know the size of the messages that are causing the problems? You can change the message size that Zimbra accepts with the following commands:
Code:
zmprov mcf zimbraMtaMaxMessageSize 51200000
zmprov mcf zimbraFileUploadMaxSize 51200000
Obviously change the file size to something that is in excess of the ones causing the problem and run the commands as the Zimbra user. If you don't know the size then just increase it to 50MB for example. You can get the defaults for the fields by running the above commands with 'gcf' (and no parameter) instead of 'mcf'