The UPN (Universal Principle Name), or as Active Directory calls it, "user logon name", are based on the IETF RFC 822, Standard for the Format of ARPA Internet Text Messages. The way active directory handles them can get a little strange sometimes. The following article has some very useful undocumented stuff about UPNs. This will prove invaluable when setting up a hosting service with multiple domains, especially for email purposes

http://theessentialexchange.com/blogs/michael/archive/2007/11/13/the-user-principle-name-and-you.aspx [updated]