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 84,105,010 websites that are SPF Customers. We know of 2,000,002 live websites using SPF and an additional 82,105,008 sites that used SPF historically and 1,198,513 websites in the United States.
Get a list of 84,105,010 websites using SPF which includes location information, hosting data, contact details, 2,000,002 currently live websites and an additional 5,135,819 domains that redirect to sites in this list. 82,105,008 sites that used this technology previously and 1,198,513 websites in the United States currently using SPF.