I just rebuilt my home server and was trying to get my e-mail back up. I'm running FC4 with Postfix. Whenever I'm somewhere other than my home network and try to send an e-mail via a software mail client, I get and error 554, Relay access denied. I didn't get this before I rebuilt my server. I was running FC3 before, but that shouldn't matter. Can anyone tell me what lines to look for in the config to make sure that I am able to relay. Basically, I just want to use the local username/password for authentication since there are only going to be a few users and one domain.

Maybe 'relay_domains' option in main.cf or tweak 'mynetworks'. These files should reside in /etc/postfix.