It is possible that is the problem, I am not doing the DNS forwarding for this domain. It is done at the isp's DNS server. However, I would think that the mail would come back as undeliverable. But, it doesn't.
The website that is running on this domain is still working just fine. So, I am inclined to think that it is not a forwarding issue. But, it certainly could be. Just to be sure I will call the isp and see if they are forwarding port 25 to the proper public address.