I did a DNS report on my server and found I had one "FAIL", which was:
"ERROR: One or more of your mail server(s) have no reverse DNS (PTR) entries (if you see "Timeout" below, it may mean that your DNS servers did not respond fast enough). RFC1912 2.1 says you should have a reverse DNS for all your mail servers. It is strongly urged that you have them, as many mailservers will not accept mail from mailservers with no reverse DNS entry. "
What may be causing this, and how do I remedy the situation?
Just as it says, there is no reverse record for the IP Address of your name server.
In other words, you know that an A record maps a hostname to an IP Address, well a PTR record maps an IP Address to a hostname. You need to contact your host and have them create a PTR record for your IP Address that matches your nameserver hostname.