SPF

• http://en.wikipedia.org/wiki/Sender_Policy_Framework
• http://homepages.tesco.net/J.deBoynePollard/FGA/smtp-spf-is-harmful.html

FAQ

===Check SPF record of a host===
dig domain.com TXT

===SPF - Returned fail===
Misconfigured SPF record or spoofed "From" address