We recently had the same behavior for MS Exchange 2003 subscribers. The original poster had the REPRO option set but did not receive their own copy.

The Exchange administrator reported that the mailbox server detected the email sent by the list as a duplicate because it had the same message ID within 1 hour. They referenced http://msexchangeteam.com/archive/2004/07/14/183132.aspx

The whole effect was broader. The original poster also copied other subscribers at the same site using BCC: All the duplicates were not delivered and they wondered whether the posting was made.

In point of fact, the list sent message was not the same as the original. It had a standard security disclaimer appended to the end that the original did not contain. I wonder whether mail with a list placed footer would be detected as a duplicate.

I will recommend the subscriber to use (ack norepro). However, this method does not allow the whole group to monitor progress like they intended.

Is there a reason the same message Id is used by Listserv?

Barry