Qmail, routing through another mailserver
If you're on a residential IP address, most modern email systems will refuse to accept mail you send to them directly. (They assume you must be a Windows machine infected with a trojan that's sending out viruses or spam.) So if you're using Qmail, and you want to be able to send mail out from your server using the mail command, you'll need to create an smtproutes control file:
ph34r# echo :real.mail.server.net >> /var/qmail/control/smtproutes ph34r# cat /var/qmail/control/smtproutes :real.mail.server.net
This tells Qmail that all outbound mail needs to be relayed through real.mail.server.net instead of being sent directly to the server which handles the mail for the recipient, thereby getting you around the RBL's (and, with many ISP's, outright blocking of port 25 outside their own backbone).