A good overview and some links: http://www.activecampaign.com/spf/ SPF Record Testing Tools: http://www.kitterman.com/spf/validate.html? Lookup SPF records: http://www.mxtoolbox.com/spf.aspx SPF Record Wizard: http://www.microsoft.com/mscorp/safety/content/technologies/senderid/wizard/default.aspx