I have the same problem. After several years of using Mail.app with our exchange server, it suddenly stopped retrieving mail from the server, though I can send. In the Connection Doctor window, it says that login the Exchange Server was successful, yet it also says "could not connect to this Exchange server."
I tried setting up an IMAP account with the same settings, without success. Of course, our network IT guy says nothing is changed at his end...
Help?