Many, if not all of my users have personal address books. If they have a name in there with no email address, my system assumes that they must be part of my domain and appends the domain name to the address. Of course, they are not users and the emails go to the dead message box.
Is there anyway to stop this?
Subject: Domain name added to email
You could try NOT holding dead mails, but passing them back to sender.
Or you could work with the users to explain that they can’t send emails to someone if they don’t have an email address for them. 
Sorry, but those are the only things I can think of. Unless you want to get really wacky, like if you have a repeat offender, you could alias the “John Doe” to your sender, so when he sends to John Doe, he receives his own message. But this is likely to confuse the user into thinking the message was sent. Which is why deliveyr failure messages (instead of holding dead messages) are the way to go.