Using older versions of Outlook for Mac in business environments
I'm up to date on 15.13.1 and password prompts have stopped. But, it wasn't without some work. Keychain does still seem to be involved and it has not been fixed (still appending the wrong domain). But, entering the username without domain of any kind was the only thing that worked for me. Could not get domain\username to work and username@domain didn't work either. This does not seem to be consistent for other users, so I would not call this clearly corrected yet.
So far, no password prompts unless I delete all keychain entries. Then the prompts come back until I enter in proper format (no extra stuff).
Let's hope this gets cleaner still in 15.13.2
By the way...:
Tested on two desktops, one laptop, different locations and different setups including VPN - all running 10.10.5 - same results
I will reiterate that this was never a problem for one desktop inside organization network, so I still suspect this is roaming or offsite issue. But, could be wrong on that.