The Sender Policy Framework is an open standard specifying a technical method to prevent sender address forgery.
http://www.openspf.org/
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Get access to data on 78,954,748 websites that are SPF Customers. We know of 2,000,001 live websites using SPF and an additional 76,954,747 sites that used SPF historically and 1,173,109 websites in the United States.
Get a list of 78,954,748 websites using SPF which includes location information, hosting data, contact details, 2,000,001 currently live websites and an additional 5,240,858 domains that redirect to sites in this list. 76,954,747 sites that used this technology previously and 1,173,109 websites in the United States currently using SPF.