Which server is bouncing the mail? DH or your own mail server? Posting the entire bounce message would be more helpful. Also, run the following from a command line and post the output:
nslookup -q=mx <domain>
Then try the same using a different DNS server, for example (assuming you're not already using Earthlink):
nslookup -q=mx <domain> ns1.earthlink.net
Using your actual domain name would be helpful as well.
If you want useful replies, ask smart questions.