Mac Outlook 2011 Error 18500

Hi.
My client is running SBS 2011/Exchange 2010.  He is running on a Mac with Outlook 2011

The mac is able to RECEIVE email just fine from the exchange server.
However, the Mac CANNOT SEND any email.  Just sending a short 1 word email, with no attachment, gives the error:

"HTTP error. The server cannot fulfill the request.
Details
----
Mail could not be sent. The message has been moved to your Drafts folder.
Error -18500"

I *really* hope someone can help!  I''m stumped.

TIA,
jim
201410-LO-Qu-021

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