I have a site with multiple e-mail domains.
Most users have all e-mail address associated with their account, so, UserA might have e-mail addresses email@example.com, firstname.lastname@example.org, email@example.com.
If they try and access their OWA, they can get in, no problems with UserA and their password.
I have a few users who are setup with only domainc.com e-mail addresses. For instance, I have userB who's username is userbtk and their e-mail address is firstname.lastname@example.org . When they try to login to OWA, i put their username in as userbtk and their password and it just keeps reprompting for the password, eventually, 3 or so tries, we get access denied error messages.
Windows 2000 Server, Exchange 2000.