Check that your login credentials actually work, by using them with an IMAP e-mail client, such as Eudora or Thunderbird. In my experience, Exchange expects usernames of the form:

The slashes are literally in the username. "DOMAIN" is your Windows NT-style domain, "login" is a Windows login who is able to access that e-mail address, "alias" is the e-mail address's alias setting in Exchange (this if often the first part of the e-mail address - before the "@" sign).