So, when you try to connect your O365 accounts via iOS Mail, you will obtain an error message. In my case, when I onboarded a user for MFA, I disabled legacy protocols such as IMAP, POP and EAS on his mailbox, because these protocols are not secured and permit to bypass the MFA. So, if you use Modern Authentication, and that you require MFA for your users when they sign in to a O365 service, and that you have disabled ActiveSync – because it is a legacy protocol – you will have some trouble to have access to your mails. Modern Authentication is a prerequisite to apply MFA on the user. Since iOS 11.3.1, the native mail app can support Modern Authentication. Problem with mail native apps with Modern Auth & Exchange ActiveSync Disabled
