SMTP Security
SPF (Sender Policy Framework): Verify that incoming mail from a domain is being sent from a server authorized by that domain’s administrators.
DKIM (Domain Keys Identified Mail): Verify that incoming email messages are authentic and have not been altered in transit. DKIM adds a digital signature to the email message header that is generated by the sending mail server.
DMARC (Domain-based Message Authentication, Reporting & Conformance): a protocol that builds on SPF and DKIM to provide email authentication and protection against phishing. Specify how their emails should be handled if they fail SPF or DKIM checks.