Using SPF, DKIM, and DMARC to prove an email came from a legitimate source, preventing spoofing.

Showing that the message, email, or post contains information that only the alleged author would know.

To authenticate means to prove that something—a user, a document, or a piece of evidence—is genuine, real, and not a forgery or a copy. It is the essential process of verifying the identity of a user, process, or device, usually as a prerequisite to allowing access to resources in an information system.