Client applications use SMTP to transfer mail to a server for submission. Client applications that connect to an SMTP server can use a number of different authentication mechanisms. In some scenarios, clients can use existing authentication mechanisms to authenticate with the SMTP server, such as the NT LAN Manager (NTLM) Authentication Protocol. However, in other scenarios, existing authentication mechanisms are unavailable or clients may not implement them. This extension provides an authentication mechanism for SMTP clients that is simple to implement.
The SMTP Service Extension for Authentication, as specified in [RFC4954], defines a service extension to SMTP, as specified in [RFC5321], where a client specifies an authentication method to the server and performs an authentication protocol exchange. This extension is one such authentication method for SMTP. It allows clients to authenticate to SMTP servers over a standard SMTP connection by passing authentication information in SMTP commands and responses.