Out Look

Hi, I am beating my head up to the wall to try figuring out why my users have a weird problem. Here is the scope: User A try to send mail or invite User B to meeting and UserB never get it, but everyone else in my company can be able to send or invite to UserB no problem. I have looked at the user A, and B mailboxes and see nothing wrong. Does it ring the bell to anyone? very appreciated your response.