MFA on Apple integrated iMail
We had a few growing pains enabling MFA/2FA on our Microsoft Office 365 online domain for iPhones using the integrated iMail app
Before you leap, check the following setting from your admin account
I suspect the default setting depends on how old your domain is.
If Modern authentication is not enabled you are in for a world of head-scratching.
Second trick is to get users to authenticate to a Microsoft App first and open Apple mail second.
Source https://techcommunity.microsoft.com/t5/identity-authentication/office-365-mfa-enabled-users-and-the-apple-mail-app-for-ios/m-p/222553