I removed a particular user from all distribution groups manually about five days ago. This user was a member of two particular groups that have other recipients. The OAB polling interval is 30 minutes, not that it really matters here. The situation is that I have an SMTP server that is not part of my Exchange organization that sends out automated email reports to these distribution groups. It sends them using a from: address that is a member of our Exchange organization. That member receives a bounce-back email indicating the member that should have been removed from the group does not exist.

I have also verified that this is the same behavior when sending an email from a webmail service like GMail or Hotmail (outside of our Exchange organization of course) to either of those distribution group addresses.

However when I send an email internally to one of those distribution group addresses everything works as expected (no bounce messages.)

Not sure why this would be happening, but also not sure how to go about diagnosing the issue. I've looked at the SMTP headers and there are no relevant clues there as far as I can tell. I think it's an Exchange issue.