One of our managers has been complaining because every time she sends a meeting request to a regular group of about a dozen people, she receives an immediate non-delivery report from an employee who is no longer with the company (not one of the group she's sending to). No big deal, I thought, as I've run into this dozens of times in the past. All I have to do is open each recipient's mailbox, and check their delegates list for the former employee, remove it, and the problem would be fixed.
No such luck this time. After reconfiguring Outlook a dozen times and opening each mailbox and examining the delegates lists, the offending entry was nowhere to be found. My next step (which probably should have been my first, but hey, I thought I knew what the problem was) was to determine which recipient was to blame for forwarding a message to the non-existent employee. I sent individual meeting requests to each person in the group, one by one, waiting for a bounce. As luck would have it, I didn't get a bounce until the very last meeting request.
According to Microsoft (http://support.microsoft.com/kb/909638/), this kind of thing can occur if delegate's mailbox object is deleted from Active Directory before it's removed from the delegate list in Outlook, which is exactly what happened here. Once the object is removed from AD, it can disappear from the delegates list, but the underlying rule that forwards meeting requests remains, which results in non-delivery reports.
The article suggests two possible workarounds. The first is one of those, "well if I had done that, we wouldn't have this problem"-type things - delete the delegate prior to deleting the object from AD. Duh. The other, and more useful solution, is to use the "/cleanrules" switch with Outlook to remove all of the mailbox's rules, including the invisible ones.
That did the trick, and eliminated the bounces. Oh, and if you encounter a similar problem, be sure to note any existing rules before using that switch, as it wipes out all rules in the mailbox, including those added by the user.