I have come accross this same issue just today.
I was using outlook 2003, the issue with outlook 2003 were the rules I had
set up to move the message reply from a meeting attendee from my indox to my
archive folder inbox. I set this rule up in order not to recieve the "I'M
over my limit" for my inbox. I had to change the rule to move a "Copy" of the
message to my archive folder inbox and leave the original message in my
inbox. Outlook uses a program called a "sniffer" to sort mail and put meeting
notices on the calendar also responces and track them. the rule I had set up
moved the message out of the inbox to quickely not allowing the sniffer
program time to sort the responce to the meeting notice.
Now if you are using Outlook 2002 and are having this problem and have not
yet insalled "Outlook 2002 post-Service Pack 2 hotfix package that is dated
September 12, 2003" do so and that should also resolve your proble.

I hope this will help.

Regards,
2ndN@ture