SPF (Sender Policy Framework) DKIM (DomainKeys Identified Mail) DMARC (Domain-based Message Authentication, Reporting and Conformance) This ensures that the letter is indeed sent from the set domain , that the content has not been counterfeited or tampered with, and that it is correct and enhances the credibility of the letter, allowing ISPs to consider your e-newsletter to be more... https://www.aucellnumbers.com/