I did this once. I can't recall how to do it anymore AND/OR it works differently in Windows 2008 than it did in Windows 2003. I recall it being an Exchange hosting feature.
I would like users to login with their email addresses instead of only with their domain name.
EXAMPLE:
User: John Doe
User logon name: jdoe@domain.local
User logon name (pre-Windows 2000): DOMAIN\jdoe
E-mail: jdoe@domain.com
I would like for jdoe to be able to login as jdoe@domain.com
Best Answer
Got it: http://www.messagingtalk.org/complete-guide-to-configure-multitenant-hosting-for-exchange-200
You need to open up "Active Directory Domains and Trusts" management console and right click on the root level, click on "Properties" Add the domain name into the UPN. This step is to allow your customer to login to the common OWA page using their own email address.