Situation: After enabling MFA, some iPhone work for a while and have a problem to open the email. It keeps popup for entering the password or saying incorrect password.
Resolution: 1. Got to Azure MFA to disable and re-enable the MFA. Wait for 10 to 15 minutes.
2. Set up two-factor authentication for your Apple ID.
3. Create app password by following this how to: How to create Microsoft app password in Office 365