I had several times on our Change 2007 server where folks have called and said that a person they emailed have gotten the email from them like 10 times. I have noticed that this only happens on some not all of them. I had a user send a email to someone at redhat.com and I noticed that the messages got stuck in the queue. I was able to clear them out so that way they would not get like 10 message sent to them. Now in the past I think someone had suggested that you set up "Send Connectors" so I thought I might try that for redhat.com. But how do you know what IP address to use for the Smart Host??
I've seen this happen in previous versions of Exchange where Antivirus software wasn't allowing Exchange to delete the queued messages. I have not seen in it with Exchange 2007, and since it stores message queues in a database it wouldn't have the same problem as before.
I would check to make sure that your firewall (and Windows firewall) are configured properly. If that all checks out, I would turn on SMTP logging and then wait for it to happen again. You can then go through the protocol log, search for the message and then see if they are being sent successfully or the recieving MTA is sending back another response.
- Joel