Exchange 2019/2016 coexistence. Outlook 2019
All mailboxes moved to 2019. I powered off 2016 to test.
Now in Outlook, Bob (bob@domain.com) is getting a credential box for test@domain.com constantly. There is no test@domain.com. It looks like Outlook is still connected but the box constantly pops back up. The same thing happened when creating Bob's account in Outlook on another computer.
Bob is the only one with this problem as far as I know.
Turning exchange 2016 back on seems to correct the problem.
Bob's OWA works fine when 2016 is powered off.

