Exchange Online email applications stopped signing in, or keep asking for passwords? Start here.

LAzevedo22 : On an affected user's iPhone, do you see a "Re-enter password" button in Settings > Mail > Accounts > [Exchange account name] ? (looks something like the image posted on an Apple forum but with Exchange instead of Gmail: https://discussions.apple.com/thread/7418609 ). For the few users in my org that hadn't already been moved to modern auth (by delete and re-add) before 15.6 became available, they could just tap that button, log in to the modern auth page that appeared (they had to correct the email address in the modern auth dialog since it had domain\[email address] instead of just [email address]), and that switched it over. These users were on 15.6.1. I did that with the users before I switched off basic with an authentication policy (their email was not popping up a password prompt since basic wasn't shut off yet). If you don't see that button, try updating to 15.7 (since I don't think 15.6.1 is available anymore).