Exchange 5.5 – Repeating e-mail problem

I am running Exchange 5.5 w/SP4 on a Windows NT 4.0 system w/SP6a. Some of my users are receiving the same e-mail message over and over again at regular intervals. It has happened several times over the last few weeks. It happens to different users recieving e-mail from different clients.

I believe it is a problem with my Exchange Server not responding back to the sending server that the message was delivered. Many times, the same e-mail comes in 1 hour intervals and then stops after a few days.

Anyone have any experience with this that can give me some ideas on what to try to resolve this problem?