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Steps to add an Office 365 account with Multi-factor Authentication enforced to work with the Mail App iOS or Later are as follows.

1. Feb 04, 2019  Thanks for your posting here, based on your description, since your Office 365 account works well on Outlook for Mac, your Office 365 account should be okay, If it doesn't work in mail app for Mac, please try the following steps to manually configure your Office 365 account on mail app for Mac to see the result. Choose Mail Preferences and go to the Accounts pane.
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1. Download the Microsoft Authenticator app onto your phone. You should have done this when you first setup multi-factor authentication for your email but if you have not you will need to do so before continuing. Please follow the steps in the below link to do so https://support.office.com/en-us/article/use-microsoft-authenticator-with-office-365-1412611f-ad8d-43ab-807c-7965e5155411#ID0EAADAAA=_Step_1
2. Now on your phone got to Settings. Then go to 'Passwords & Accounts'. Now under accounts go to 'Add Account' .
3. Select Exchange.
4. Now enter your email address and select 'Next'.
5. A popup that says something to the effect 'Sign in to your 'whatever domain name goes here' Exchange account using Microsoft?' Select 'Sign in'.
6. Now a login.microsoftonline.com screen should come up. Enter the password for your email address and select 'Sign in'.
7. You should now see a screen that says 'Approve sign in request.'

Before continuing, I have to mention that before iOS 12.3.1 if you selected 'Receive notifications for verifications' when you set up the Mircosoft Authenticator, Approving the sign on would not work. You would have to select 'Sign in another way' at the 'Approve sign in request' screen. Then select 'Use a verification code from my mobile app'. Open the Microsoft Authenticator app and make a note of the code. Make sure that you have enough time to enter it in before the timer on the code runs out. Select 'Verify'. With iOS 12.3.1 all you have to do is open the Autenticator and select 'Approve' if you chose 'Receive notifications for verifications' when you set up the Microsoft Authenticator.

Mac Mail Office 365 Settings

8.Select which options you want available to you on your phone like mail, contact, calendar, etc. and select 'Save'.

Add Office 365 Email To Mac Mail App

You should now be able to retrieve email from an Office 365 account with Multi-factor Authentication enforced using the Mail app.