Hi,
Let’s check the answer in the following thread:
http://social.technet.microsoft.com/Forums/exchange/en-US/8ecae470-e4c3-41ee-b9fc-c339b1e27aaa/exchange-2013-server-outlook-login-with-user-principal-name?forum=exchangesvradmin
“If you want to use the user principal name when connecting to users with outlook, the email domain need to be the same as the Windows domain.
If your email domain name is not the same as windows domain, there is a work around that adding the email domain as a UPN suffix in Active Directory, then changing the user's UPN suffix to match email.”
When the client tries to connect to your messaging environment, the client locates the Autodiscover service on the Internet by using the right side of the user's email address that was entered. Notice that, for the Autodiscover service to function correctly,
this must be the user's primary SMTP address.
For more information, you can refer to the following article:
http://technet.microsoft.com/en-us/library/jj591328(v=exchg.141).aspx
Thanks,
Angela Shi
TechNet Community Support
