While spammers certainly don't worry about conforming with RFCs, it should be noted that rejecting email based on the DNS is a specific violation of the relevant RFC, RFC-5321, section 4.1.4, which states:

An SMTP server MAY verify that the domain name argument in the EHLO command actually corresponds to the IP address of the client. However, if the verification fails, the server MUST NOT refuse to accept a message on that basis.

That said, it is common practice to do this anyway. The reality is that you will not have reliable email delivery unless the hostname your server issues in the EHLO command resolves to the servers IP address and the IP address resolves back to that same name.

Try BZflag (www.bzflag.org).

Its open source and available for Linux/Windows/Mac and probably others.

Its conceptually simple, and therefore pretty quick to get started, but there are always new tactics/counter-tactics to learn.

Being open source, there are cheaters. But I've found this to be a minor problem. Most players seem to be honest and when you do encounter someone that appears to be cheating, just switch to a different server.

Its very sensitive to latency, so you will likely find that some servers work better for you than others. Each server has its own combination of rules/options, so you should try out several servers anyway, just to see what you like.

Its Free! Give it a try.