If just enabled a Conditional Access Policy blocking legacy authentication to Exchange Online, enabled Security Defaults, or Microsoft disabled it for your tenant, you might see some Apple Mail clients not connecting anymore.
This issue is happening because the profile might be still configured to use Exchange ActiveSync to connect to Exchange Online, and EAS (along with other legacy protocols) will be retired in October 2022.
Apple supports an automatic switch to modern authentication for its profiles, but only if it was freshly configured after iOS 12.
Unfortunately, it seems that backing up and restoring profiles does not trigger the switch to modern auth, so if you moved to a new iPhone and didn’t reconfigure the profile manually, you’ll need to remove and recreate it.
UPDATE 16.06.2022:
Apple will add support for the automatic migration to modern auth in iOS 15.6. Once you update your Apple device, the Mail app will use the saved credentials to establish a new authentication flow. From that moment onward, you’ll authenticate to Azure AD (Microsoft online Identity Provider) and get a new OAuth access token. The “old” stored credentials will then be removed. The process is fully transparent to users.
Read the full announcement here: Microsoft and Apple Working Together to Improve Exchange Online Security
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