http://wiki.zimbra.com/index.php?title=Improving_Anti-spam_system#SPF
http://wiki.zimbra.com/index.php?title=Ajcody-MTA-Postfix-Topics#What.27s_Your_SPF_Records_Say.2C_When_Getting_.22does_not_designate_74.x.x.x_as_permitted_sender_Errors.22