Sender Policy Framework (SPF)
Prevent others from sending spam using your domain name.
Frequently asked questions about SPF:
http://spf.pobox.com/faq.html
Check SPF records:
http://www.dnsreport.com/
http://www.anti-spamtools.org/SenderIDEmailPolicyTool/Default.aspx
http://www.dnsstuff.com/pages/spf.htm