Outlook meeting request

I have a user(A) who is facing this problem
1. User (A) sends a meeting request to user (B)
2. User (B) gets the meeting request and
accepts it
3. User (A) gets a response as accepted
4. User (B) gets an Undeliverable email saying
The following recipients cannot be reached
and lists User (C)
5. User (C) was working with User (A) and left
the company 5 years ago
6. User (c) profile does not exists on the
exchange
7. User (A) has no delegates added on her
profile
8. when ever User (A) sends a meeting requests
she recv the same msg and calls from the users
she send saying they recv this msg

Any idea how to get this sorted.
Thanks