I can't add an O365 email to MS Outlook

Question

Hi all,

"myaccount@mydomain.com" and "user@mydomain.com" are Office 365 accounts .

My PC and user's PC are Windows 10 , with Office 2016 standard

I can add "myaccount@mydomain.com" to MS Outlook on my pc and user's pc

I can't add "user@mydomain.com" to MS Outlook on my pc and user's pc but I can access it from the web.

I go to https://testconnectivity.microsoft.com --> Office 365 --> Outlook Connectivity and test "user@mydomain.com" with Modern Authentication (OAuth) credentials successfully.

How can I troubleshoot it ?