[mmorse]

good to hear - another trick I've found is setting the zimbraMtaMaxMessageSize 10% larger than the zimbraFileUploadMaxSize.

Say both are set to 10MB (and you tell everyone they can send 10MB attachments):
- An 9MB attachment and 100KB message body text. The attachment may be less than the 10MB zimbraFileUploadMaxSize, and so it'll get accepted by the file upload system. But when it's converted into a MIME message it ends up base64-encoded. Then add 100K of regular body text, and the resulting message (say 10.3MB or so) is then handed off to postfix, which checks zimbraMtaMaxMessageSize and promptly rejects the message if it's over the limit of 10MB.

Of course set zimbraMtaMaxMessageSize larger if you want to allow emails with multiple large attachments.
For your situation an upload size of 512MB and message size of 1GB is probably more useful. - Really hope neither are ever reached, because you would perfer that they'd use a file server instead.
(And of course don't forget the registry key for the import wizards.)
Also, there's a spamassassin setting to not scan/spend cpu resources on really large messages that are over x value in size - adjust accordingly.