(This could be relevant to you due to a recent change in Microsoft Security settings within Office 365. We are still gathering information, but it appears that the IOS mail app may not support something called Modern Authentication that is now required. Appears to affect those that may have added an "exchange" account some time ago. This does not affect those that were using the recommended Microsoft outlook app for iOS.)
If you are using an iPhone and the inbuilt iOS app then can you do the following:
On phone issues like this the normal route is to remove the phones mail account and re-add it.
- On your iPhone go to settings (cog)
- Goto mail / accounts
- remove the existing account
- Using the Microsoft Outlook app (preferred*) go through process of creating a new Office 365 / Microsoft 365 mail account with the previously supplied details.
- Download from Apple Appstore and it should prompt for login details however steps are listed in the link below
- Instructions for this can be found here
- That should be you. But any issues then let me know.