We have an internal server that manages both the mail and the domain accesses, there are personal accounts enabled for the internal network only, and group profiles that send/receive on the public side.
Every user has an account and password to enter the domain (lan) but it's not directly related to the mailbox. For example I could have more than one user allowed to read the same mailbox.
Actually to connect to a mailbox is not needed an account/password and it seems with outlook this is not a problem, while other apps require it obligatorily