Email authentication, or validation, is a collection of techniques aimed at providing verifiable information about the origin of email messages by validating the domain ownership of any message transfer agents (MTA) who participated in transferring and possibly modifying a message. The original base of Internet email, Simple Mail Transfer Protocol (SMTP), has no such feature, so forged sender addresses in emails (a practice known as email spoofing) have been widely used in phishing, email spam, and various types of fraud. To combat this, many competing email authentication proposals have been developed, but only fairly recently have three been widely adopted – SPF, DKIM and DMARC. The results of such validation can be used in automated email filtering, or can assist recipients when selecting an appropriate action. This article does not cover user authentication of email submission and retrieval. Rationale In the early 1980s, when Simple Mail Transfer Protocol (SMTP) was designed, it provided for no real verification of sending user or system. This was not a problem while email systems were run by trusted corporations and universities, but since the commercialization of the Internet in the early 1990s, spam, phishing, and other crimes increasingly involve email. Email authentication is a necessary first step towards identifying the origin of messages, and thereby making policies and laws more enforceable. Hinging on domain ownership is a stance emerged in the early 2000. It implies a coarse-grained authentication, given that domains appear on the right part of email addresses, after the at sign. Fine-grain authentication, at user level, can be achieved by other means, such as Pretty Good Privacy and S/MIME. At present, digital identity needs to be managed by each individual. An outstanding rationale for email authentication is the ability to automate email filtering at receiving servers. That way, spoofed messages can be rejected before they arrive to a user's Inbox. While protocols strive to devise ways to reliably block distrusted mail, security indicators can tag unauthenticated messages that still reach the Inbox. A 2018 study shows that security indicators can lower the click-through ratio by more than ten points, 48.9% to 37.2% of the users who open spoofed messages. Nature of the problem SMTP defines message transport, not the message content. Thus, it defines the mail envelope and its parameters, such as the envelope sender, but not the header (except trace information) nor the body of the message itself. STD 10 and