Sender Policy Framework is designed to help you avoid the mail you sent out to be marked as spam.

SPF has a simple concept. You crate a TXT DNS record on you domain in which you specify all of the servers that are allowed to send email in your name (with your email address as the origin). The servers that receive mail can check the DNS record of your domain and if the mail is coming from the server that is not specified in the DNS record the chances are good that somebody is spoofing your email address.

A samepl DNS record can look like this:

example.net.  TXT  "v=spf1 mx a:pluto.example.net include:aspmx.googlemail.com -all"

This record states that all mail coming from your domain can come from three sources: IP addresses that are registered as MX servers for your domain, server with an A record pluto.example.net and all servers that are considered valid by gmail.com service.

You can find more information about SPF on http://www.openspf.org/ .