I didn't find anyone posting about this, so I'm guessing no one is attaching large attachments? For me, at least, I'm using the latest Zimbra stable under Ubuntu LTS.
If I set:
zmprov mcf zimbraFileUploadMaxSize 1024000000
zmprov mcf zimbraMtaMaxMessageSize 1024000000
then attaching files greater than about 200MB causes Zimbra to crash. A restart is required before people can connect to the imap server again.
As for the briefcase, it just won't take the attachment. No error message, it just doesn't attach.
Any ideas? I guess I should open a bug report?