SMTPS refers to a method for securing SMTP

Simple Mail Transfer Protocol

Simple Mail Transfer Protocol is an Internet standard for electronic mail transmission across Internet Protocol networks. SMTP was first defined by RFC 821 , and last updated by RFC 5321 which includes the extended SMTP additions, and is the protocol in widespread use today...

 with transport layer security

Transport Layer Security

Transport Layer Security and its predecessor, Secure Sockets Layer , are cryptographic protocols that provide communication security over the Internet...

. It is intended to provide authentication

Authentication

Authentication is the act of confirming the truth of an attribute of a datum or entity...

 of the communication partners, as well as data integrity

Data integrity

Data Integrity in its broadest meaning refers to the trustworthiness of system resources over their entire life cycle. In more analytic terms, it is "the representational faithfulness of information to the true state of the object that the information represents, where representational faithfulness...

 and confidentiality

Confidentiality

Confidentiality is an ethical principle associated with several professions . In ethics, and in law and alternative forms of legal resolution such as mediation, some types of communication between a person and one of these professionals are "privileged" and may not be discussed or divulged to...

.

SMTPS is not a proprietary protocol and not an extension of SMTP. It is just a way to secure SMTP at the transport layer.

This means that the client and server speak normal SMTP at the application layer, but the connection is secured by SSL or TLS

Transport Layer Security

Transport Layer Security and its predecessor, Secure Sockets Layer , are cryptographic protocols that provide communication security over the Internet...

. This happens when the connection is established before any mail data has been exchanged. Since whether or not to use SSL or TLS is not negotiated by the peers, SMTPS services are usually reachable on a dedicated port of their own.

Originally, in early 1997, the Internet Assigned Numbers Authority

Internet Assigned Numbers Authority

The Internet Assigned Numbers Authority is the entity that oversees global IP address allocation, autonomous system number allocation, root zone management in the Domain Name System , media types, and other Internet Protocol-related symbols and numbers...

 registered 465 for SMTPS. By the end of 1998, this was revoked when STARTTLS

STARTTLS

STARTTLS is an extension to plain text communication protocols, which offers a way to upgrade a plain text connection to an encrypted connection instead of using a separate port for encrypted communication....

 has been specified. With STARTTLS, the same port can be used with or without TLS. SMTP was seen as particularly important, because clients of this protocol are often other mail servers, which can not know whether a server they wish to communicate with will have a separate port for TLS. The port 465 is now registered for Source-Specific Multicast

Source-specific multicast

Source-specific multicast is a method of delivering multicast packets in which the only packets that are delivered to a receiver are those originating from a specific source address requested by the receiver...

audio and video.

SMTPS will nevertheless continue to be offered on port 465, as well as on the message submission port 587 defined by RFC 4409.