SPF (Sender Policy Framework): This protocol allows the domain owner to specify which mail servers are permitted to send email on behalf of their domain. DKIM (DomainKeys Identified Mail): DKIM adds a digital signature to the email headers, which can be verified by the receiving server to ensure the email has not been altered. DMARC (Domain-based Message Authentication, Reporting & Conformance): DMARC builds on SPF and DKIM by providing a way for domain owners to publish policies on how emails that fail authentication checks should be handled.