Users can logon to a Windows 2000/2003 domain using two different logon names.
1. You can use your down-level (pre-Windows 2000) name like abc-jsmith or bcd-jsmith.
2. You can also use your User Principal Name (UPN) such as jsmith@abc.com or jsmith@bcd.com.
By default the user has the UPN suffix of `@DefaultDomain`. You can create multiple UPN suffixes and provide your users with different UPN names like jsmith@abc.com or jsmith@bcd.com instead of abc-jsmith or bcd-jsmith. Here is demonstrated how to configure and use multiple UPN (User Principle Names).