Editing Simple Mail Transfer Protocol (section)

====SMTP-AUTH====
{{Main| SMTP Authentication}}
The SMTP-AUTH extension provides an access control mechanism. It consists of an [[authentication]] step through which the client effectively logs into the [[mail transfer agent|mail server]] during the process of sending mail. Servers that support SMTP-AUTH can usually be configured to require clients to use this extension, ensuring the true identity of the sender is known. The SMTP-AUTH extension is defined in {{IETF RFC|4954}}.

SMTP-AUTH can be used to allow legitimate users to relay mail while denying relay service to unauthorized users, such as [[spam (e-mail)|spammers]]. It does not necessarily guarantee the authenticity of either the SMTP [[envelope sender]] or the {{IETF RFC|2822}} "From:" header. For example, [[E-mail spoofing|spoofing]], in which one sender masquerades as someone else, is still possible with SMTP-AUTH unless the server is configured to limit message from-addresses to addresses this AUTHed user is authorized for.

The SMTP-AUTH extension also allows one mail server to indicate to another that the sender has been authenticated when relaying mail. In general this requires the recipient server to trust the sending server, meaning that this aspect of SMTP-AUTH is rarely used on the Internet.{{Citation needed|date=October 2019}}