Had a similar problem with Entourage: 13.0.3 Web services edition and Exchange 2007 SP1
Users got:
ERROR
HTTP Error. The server cannot fulfill the request.
Explanation
Mail could not be sent. The messege has been moved to your Drafts folder.
Account name: “xxxxxxx”
Error: – 18500
Had to edit web.config (…:MicrosoftExchange ServerClientAccessexchwebews) and add <httpRuntime maxRequestLength=”30000″ /> after <system.web>, restarted IIS and that actually fixed it.
you are awesome.
thanks