Hi @MasterBlaster ,
Agree with Andy that from the quarantine reason "AadBlockDuetoAccessPolicy", it's most likely to be related to the Conditional Access policy.
Additionally, I found the following thread which discusses a similar situation. According to the comments there, it was resolved by removing the account from the device first, then approving in exchange quarantine and add the account again in the native Mail app on the device:
Issue with security defaults - activesync clients get quarantined
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