I sent an email a while back, and it has been sitting in the queue for over an hour now with the following error. any ideas on how to correct the problem?
DNS problem. Fix DNS, this problem goes away.
For mail to work reliably, you need both a forward record (mydomain.com needs to resolve to your IP address) and a reverse record (your IP address needs to resolve to something–doesn’t matter what name, as long as the name it resolves to also resolves back to your IP…usually your hosting provider handles this).
See the DNS troubleshooting section of the Webmin documentation for some hints on where to start figuring out the problem: