MSN & Hotmail re-sent messages

My users are reporting that emails sent up to 4 months ago are being resent to MSN and Hotmail clients. Group mails are also being resent to the entire group if a member of the group uses MSN or Hotmail. The headers indicate that the email was held in a queue somewhere on the MSN side and just released. Any one else seen this weirdness or have any ideas???
Thanks,
JC