Ok, in short... I am running Redhat 9.0 with kernel 2.4.20. I wrote an email to a friend of mine via the sendmail service, and I got this message back http://postmaster.info.aol.com/errors/421dnsnr.html It says that I dont have a reverse DNS... What is a reverse DNS and how can I make it so I no longer receive this error? As far as I know, reverse DNS is where an IP Address is associated with a name such as linux.net or something of the sort. Or if you put in 192.64.5.34 you come out with a name such as linux.net.,.. Am I correct in my assumption? Ok, so my main question.... how do I fix this issue? Thanks you!!

DragonM15

This is an anti-spam measure. Your mail server won't be accepted without proper MX and RMX records. You cannot fix/make these records yourself, its up to your ISP (in most cases).
Workaround is to route outgoing mail thru your ISP's smtp server.

it means that aol won't forward your mails....
aol mail servers check that the ip address of the sender server is registered as a MX record in some DNS server, ie that it is a known mail server on the internet.
since i believe you use broadband, when the aol mail server gets your mail, it asks who's in charge of this IP (which is reverse dns, instead of asking the ip associated to a name, it asks the name associated to the ip)... your provider answers "I am, and this is an IP i give to my customer, which means anyone can have it". This answer doesn't suits aol, so they drop your mail.
that's a common thing used to fight against spam, and what's worse, that's not a very efficient method (lot's of false positive - like you).
there's nothing much you can do against that... i don't think that your provider will accept to change its dns configuration for you, poor lonesome end-user.

Yes, I realize it is an anti-spam issue, however what if I am my own ISP and DNS? I am looking in my /var/named/ folder, however something must not be right in here... I can copy the source into here if needbe.

Thanks,
Dragon

Well, wherever your domain is registered, you have to make the MX record there. This ensures mail sent to your_domain finds your box. If you have authoritative DNS running you can probably make that RMX record yourself and send mail directly.

Ok, I have that problem fixed.... it was a simple matter of changing an IP around.... I got that fixed... now I was curious if I could do that same thing with my mail (The reverse DNS Check) and how I would do this? I am using sendmail. Any Ideas?

Thanks,
DragonM15