Disabled user account receiving meeting request!

User1 is using Outlook 2003. When any user tries to send user1 a meeting
request, user1 receives it, BUT any user sending the meeting request gets a
message back (delivery failure) saying that ANOTHER user's address isn't
recognized. Now, this unrecognized user address is a user whos account was
disabled and her mailbox removed. This user once had permissions to User1's
calendar. I removed this user from all of User1's folders and removed her
from all groups. Why is "any user" getting a delivery failure message, even
though a request isn't even being sent to this disabled account?