Got a weird one for you fine folks. I have a single user who has recently started getting prompted for credentials when launching Outlook. He is on Outlook 2013 but I can reproduce the problem on a different workstation with Outlook 2007. The correct login format is domain\username, but when Outlook launches it's trying to use firstname.lastname@example.org to authenticate. Once he's supplied the correct credentials Outlook is happy for the remainder of the session until he reboots.
I first cleared his credential store and created a new profile from the Mail snap-in. I noticed that when entering his username and selecting 'check name', it resolves to email@example.com. This does not happen for any other user. Where it gets weird is if I enter gibberish into the username field, it obviously fails....but when I enter his username again in the prompt which comes up following the 'name cannot be matched to a name in the address list' notice and then 'check name' it correctly resolves to his underlined domain username. Afterward, using that profile, he's good until the following restart when Outlook again assumes the firstname.lastname@example.org for authentication.
I've checked out the attribute editor and compared him against another staff member and haven't seen any obvious discrepancies. Autodiscover tests are all successful and no other user has this problem. This feels like a value got flipped and I'll have to use ADSIedit to correct it, but I don't know what to look for. Suggestions? Thanks!
Outlook 2013 (reproduced problem on Outlook 2007), Exchange 2007, Server 2008 (not R2) domain.