Old UPN reappears, causes havoc

I have a very unusual problem – at least I have never seen it before, and I’ve been administering and using Outlook and Exchange since the beginning. Here’s what’s happening: I have a mailbox that was created as john.smith@old-domain.com. Then I retired old-domain.com and changed the UPN, the login, and all the email addresses to *@new-domain.com. That was several years ago now.

All went well for the longest time, but after the Outlook update to 11629.20196 I started noticing that the mailbox name as shown in the profile and in the left-hand pane of outlook would revert to john.smith@old-domain.com. At which point Outlook becomes unusable; gives an error about not being able to log in, throws a system resource error message, and dies. I can’t run Outlook again until I remove that account from the profile. I add it back, redownload the ~8G mailbox, and it works again for a little while, a few days typically.

Another oddity: this only happens on my main desktop. Both it and my laptop run the same version of Win10 and the same version of Office, but it’s never happened on the laptop. Things that make you go hmm.

My hosting vendor suggested putting in the registry entries that fine tune Autodiscover (ExcludeLastKnownGoodURL, ExcludeExplicitO365Endpoint, etc) and I did it in spite of my skepticism any of them had any bearing on this, to no effect. I tend to think the old UPN/address is buried somewhere deep in AD and needs somebody who knows what they’re doing to hunt it down and change it. I do not have ADSIedit access, and neither (I believe) does the tech who recommended the registry entries. Somebody somewhere does, but I wanted to cast around for ideas or other people’s experience before I go shaking that tree.

PS: I’m at the point of thinking I should delete the mailbox altogether, recreate it, and reimport the contents. But boy is that going to be a PITA, because it has a ton of SMTP aliases and the vendor’s UI for adding/editing them is clunky. Not to mention as a general principle I like to fix things with a gentler touch than a bulldozer.