I have always had trouble sending LARGE attachments using Outlook 2011; however, I have recently started to be unable to send even SMALL attachments (~300 KB). 

When I try to send to send the message, I get the error:

HTTP error. The resource is not found. 

The message is then placed into my Drafts folder.

I am using:

Outlook 14.4.5

Mac OS X 10.9.5

Of note,  I am still able to send EXTREMELY small attachments (5-10KB), but this is hardly useful.

I would strongly appreciate any suggestions!