So we use Exchange Online Plan 1 and Outlook 2016. After setting up Multi-factor Authentication for one of my email accounts I found that after a couple of days Outlook started popping up the password prompt for the account.
No matter how many times I tried it wouldn’t accept the “app password” i created here or the normal password for the account. After a bit of digging I found this article which suggested adding the following registry key which solved the problem for me.
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\Identity DisableADALatopWAMOverride dword value 1