Entourage - HTTP Error. The server cannot fulfill request. Code 18500

HTTP Error. The server cannot fulfill request.
Code 18500
Text Mail could not be sent. The message has been moved to your Drafts folder.

Solution:
The issue is caused by the ASP.net post size limit in the IIS configuration. You are most likely try to send an attachemnt that is bigger then the "httpRuntime maxRequestLength".

Login to your CAS server(s) and access C:\Program Files\Microsoft\Exchange Server\ClientAccess\exchweb\EWS.
Edit the web.config file and increase the limit (in KB) for:

“httpRuntime maxRequestLength=”XXXXX”

*IMPORTANT Make sure that you set this to around double (or more) the size of the actual attachment limit or size that you try to send.