Microsoft resolved the issue with Mac clients inability to send some attachments.
The official report:
Closure Summary: On Tuesday, December 03, 2013 at approximately 3:39 AM UTC, Microsoft received reports of an issue in which some customers using Mac Outlook or Entourage may have been unable to send messages exceeding a size of 6 megabytes. Microsoft engineers deployed an update across the affected environment which remediated impact. The issue was successfully fixed on Wednesday, December 11, 2013 at 5:00 PM UTC.