Had a similar problem with Entourage: 13.0.3 Web services edition and Exchange 2007 SP1
HTTP Error. The server cannot fulfill the request.
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.