POP3 Service with Active Directory authentication
I experienced a problem trying to use Windows 2003 POP3 Service with Active Directory account...
It worked properly when the account was created manually account from the POP3 Service Console, when I tried to use the already existing account it gave me -ERR Logon Failure even if the event viewer show the logon/logoff successfully events.
I tried to reverse engineer the logic and I searched for some link between the User Account and the Mailbox, I didn't find any text files or registry keys so I looked into Active Directory DB with adsiedit.
Finally I discovered that all the account needed the UserPrincipalName equal to the e-mail address.
The new accounts were created with
UserPrincipalName = firstname.lastname@example.org
and the old account had
UserPrincipalName = email@example.com.