Have one user that if you accept appointment another user replies?
 

Have one director that when she sends users an invite when the user accepts
the invite they get a response from someone else. This person use to be their
assistant. The assisant has now left the company and the user account and
mailbox have been removed and you receive a message that this person is not a
valid user. This person has no rights to the directors calendar or mailbox
and is not listed as a delegate.
Any ideas?