On the other hand, that message was the 1st time I've seen all the pieces put together.
I was under the impression you had no MX records and you were not running an inbound server.
My mail address DOES have an MX record in register.com's DNS. However, it's an external DNS to my provider; check the IP address and it doesn't show up as the name(s) I use.
We've just hit into "who owns the block"? Who is the authoritative DNS server for the block?
In order to do a reverse DNS, you've got to start with that.
Who's name DOES show up when you do a reverse?
I seem to recall when my company does some complex DNS setups, dealing with other company's servers being dropped into our address space, we needed to setup some type of DNS delegate structure to allow it to work. Got nasty.
Let's get back to the email server conversation.
Your's connects to the other one.
Him: Who are you?
You: mail.whatever.net
Him: (does reverse DNS) - Liar! <breaks connection>
This is a perfectly reasonable response.