Our organization has quite a few Office365 accounts and we're having an odd problem with one of our users.

His Office for Mac 2011 Outlook client will not send large attachments.  It used to work until about a week ago.  What happens is it shows the "send" progress and 1/3-1/2 way through it stops trying to send the message and places the email in the drafts folder.  Error code -18500 (one of the errors)

Sending text based emails works fine.  He has no problems receiving large attachments.

Does anyone have any advice that can help remedy the situation?