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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.

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1) What are the permissions on the DL? Are there any restrictions on who may send to it? 2) Do you have a spam filter that checks against AD or an LDAP server for valid recipients before relaying the message to your Exchange server? –  HopelessN00b Jun 21 '12 at 15:27
@HopelessN00b 1) The permissions on the DL are "Accept Messages from All Senders" and do not require auth 2) Yes, there is a Spam filter that sits between. It is actually the MX of record, then forwards the mail along to a subdomain (the send to address is user@domain.com, gets routed to a third party service, then service routes to user@sub.domain.com in a split-domain setup –  tacos_tacos_tacos Jun 21 '12 at 15:36
Well, without knowing more about your environment, I can't really give specific advice, but it seems to me that the spam filter is the most meaningful difference between internal and external emails, so that's what I'd look at first to explain the different behavior. Check its LDAP/AD lookup settings, make sure that's current and correct, then dig into the Exchange message trace/audit logs, and the spam filter logs and hopefully you're logging enough detail to figure out exactly what's causing the bounce. –  HopelessN00b Jun 21 '12 at 16:10

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