POP3 login issue with Exchange 2013
A user cannot login to pop3/imap/owa using their email address
Guest post from Mike from http://cdnhost.ca/ hosting.
Problem: When you touch any of the account properties (quotas, retention, archive, etc) using ECP it will automatically change the UPN back to the firstname.lastname@example.org.
Fix: login to your domain controller and run ADSIEdit.exe, connect to your DC and then browse for the login in question, right click it and choose properties. Down near the bottom you’ll see “UserPrincipalName” and it will be set to email@example.com. Simply edit this entry and change it back to their SMTP email address and save.