I have added other exchange accounts fine, any reasons that could cause this?
Did you mean you can add accounts in other tenants?
And what IOS version are you using?
According to this link: Integrate Apple devices with Microsoft Exchange
(Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.)
iOS 11 or later, iPadOS 13.1 or later, and macOS 10.14 or later support Microsoft Modern Authentication workflows of Exchange online tenants.
If the IOS client supports modern auth, after entering the email address you would be redirected to a new page like this:
After entering the credentials, you would see a promotion like:
If you are using some previous versions, it may be possible that the mail app does not support modern auth.
And if your tenant has basic auth for activesync disabled (by default), I suppose it may be the cause of this issue.
You may use this self-help tool to have a check if basic auth for activesync is disabled.
If this is the case, you may either need to update your ios version to use modern auth, or enable basic auth for activesync in your tenant (which may be permanently disabled in all tenants on October 1. 2022).
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