I have a domain (jaymefall.com). I am hosting the domain from my Linux computer. I have a name server running. My linux computer has two network cards, one gets an IP via DHCP from my DSL and the other acts as a gateway so I can use the internet connection from my networked Windwos XP computer using IPtables.

Everything works fine except that I setup a mail server the other day and I am trying to use it from my Windows XP networked computer. My mail servers are mail.jaymefall.com. I can receive mail just fine, but when I try to send a message I get the following message from Outlook Express:

"The message could not be sent because one of the recipients was rejected by the server. Account mail.jaymefall.com, Server: "mail.jaymefall.com" protocol SMTP, server response 550.5.7.1. Relaying denied. IP name lookup failed (192.168.10.1) port 25, secure(ssl) no, server error. 550 Error number 0x800ccc79."

I am guessing that for some reason it cannot do a reverse lookup on 192.168.10.1 (which is the internal IP address of my Windows XP computer. There must be something that I have to add to my name server config to get it to do this. Does anyone have any idea what I might have to do?

The problem isn't with your nameserver.

Sendmail prohibits relaying (which means accepting emails for delivery from unknown hosts) by default to stop spammers from forwarding email through your box.

This is a good thing.

If you are using postfix you can set it up to allow relaying from your Windows machine in the main.cf config file.
It's probably somewhere like /etc/postfix or /etc/sendmail.

Make sure you adjust iptables to DENY incoming packets with a source address of 192.168.x.x on your external NIC.