This may be some simple fix that I overlooked. I attached a file exceeding a size limit and the email was bounced back with this error:
I emailed a file from my company address to my zimbra box.
The e-mail system was unable to deliver the message, but did not report a specific reason. Check the address and try again. If it still fails, contact your system administrator.
< outbound5-blu-R.bigfish.com #5.0.0 X-Postfix; message size 13234677 exceeds size limit 10240000 of server mail.example.com[xxx.xxx.xxx.xxx]>