UnknownOutgoingEmailIntegrationError while configuring the mailboxes

Hi experts,

We are experiencing issues while trying to configure mailboxes for few users and queues. It works for many other users and queues but for some it fails giving errors like:

-          Email Server Error Code: ExchangeOutgoingAutoDiscoverFailure

-          Email Server Error Code: UnknownOutgoingEmailIntegrationError

Its CRM Online 365 (8.2) and We are using the same Email Server Profile for all the mailboxes.

Any idea what could be the reason?

Thanks