IMPOSSIBLE TO VERIFY ACCOUNT INFORMATION FROM THE MAIL APP EXCHANGE - IPHONE TRYING TO SIGN IN

Hello, 28GREKA 

First of all, I'll suggest to check in EXO that the configuration for the user is correct:

Please check also if the device is blocked for the user. You can do it connected to EXO PS running the following cmdlt:

Get-CASMailbox email address removed for privacy reasons | fl activesync*

If the device is ActiveSyncBlocked, copy the device ID and paste it in the following cmdlt.:

Set-CASMailbox -Identity mailto:email address removed for privacy reasons -ActiveSyncAllowedDeviceIDs @{add='F2BC03A83CCA6C98'}

If all is correct, then probably the issue is device related.

I'll try to, instead to add the account on the ios mail app, add the O365 account to the device settings-accounts. Then you can select which data you would like to sync and test that. That worked for me in the past.

Hope this helps. Good luck 😉