My wife sent my son and I an email, and the address for my son got munged somehow. After an evening of sending test messages it appears that when I get my copy of the email through a forwarding service and the forwarding service fixes my email address it also "fixes" my son's address. His address is to a domain at home, and the home system uses dynamic DNS, and the hosting service has a CNAME alias record to the dynamic address at dyndns.com. So I think the CNAME alias is what is causing the problem.
So my solution is to use the dynamic address service at Namecheap. Since namecheap doesn't have a Macintosh client, I had to go looking for something to use. I found ddclient, which is perl based so therefore compatible.
Dynamic DNS client:
Now, there are a couple of problems. One is that I host several domains on the same Snow Leopard Server. So I found a patch that allows ddcleint to update several domains.
Patch for multiple domains:
The next problem is how to get ddclient running on system start.
Startup items on OS X:
Set it all up and tested. Got bit a couple of times by the cache. It's really important to delete it when changing the config and testing to see if the provider is getting updates.
It did indeed fix the problem.