Using older versions of Outlook for Mac in business environments
not sure why this solution is not being spread around more. But to fix this issue, what you have to do it add a "" before your username. for example:
Username: \domain\username
Password: whatever your password is
Make sure you save to your keychain after. This has worked for me. I'm not sure where I found this, but it was a lifesaver. Hope it helps for you!