The Sender Policy Framework is an open standard specifying a technical method to prevent sender address forgery.
http://www.open-spf.org/
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Get access to data on 182,303,272 websites SPF Customers. We know of 140,396,089 live websites using SPF and an additional 41,907,183 sites that used SPF historically and 9,424,209 websites in the United States.
Get a list of 182,303,272 websites using SPF which includes location information, hosting data, contact details, 140,396,089 currently live websites and an additional 5,612,691 domains that redirect to sites in this list. 41,907,183 sites that used this technology previouslyand 9,424,209 websites in the United States currently using SPF.
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