What Is SMTP?

Date First Published: 23rd March 2022

Topic: Computer Networking

Subtopic: Internet Protocols

Article Type: Computer Terms & Definitions

Difficulty Level: 5/10

Learn more about what SMTP is in this article.

Not to be confused with SNMP (Simple Network Management Protocol).

Short for Simple Mail Transfer Protocol, SMTP is a protocol used for transferring outgoing mail from one server to another. It is only used for sending email, not receiving. Therefore, the server that the email is sent to is known as an SMTP server.

SMTP is an application-layer protocol that is often used in combination with other protocols, such as POP3 and IMAP for downloading the message from the server. These protocols do not work in the same way. POP3 downloads the email message to the device and deletes it from the server, whilst IMAP downloads the message to the device and keeps the message stored on the server and synchronises it across multiple devices.

Note:

Email applications, such as Outlook, Yahoo, and Gmail make use of SMTP.

Security

SMTP itself lacks encryption and the emails messages are sent in plain text. It could be intercepted by anyone monitoring the connection. However, a secure version of SMTP, called SMTPS (Simple Mail Transfer Protocol Secure) was released in early 1997, when IANA registered port 465 for SMTPS. SMTPS is secured by SSL/TLS, which encrypts messages sent between clients and servers when the TCP connection is made.

History

Before SMTP was released, different versions of instant messaging were used in the 1960s. Users communicated through systems developed for certain mainframe computers. The increase of connected computers, such as the ARPANET of the US government led to standards being developed to enable the transferral of messages between different operating systems.

SMTP comes from two standards that were introduced in 1971. An example is the Mail Box Protocol. The development of the Mail Box Protocol was resisted, but it was discussed in RFC 196 as well as other RFCs and the SNDMSG program. In accordance with RFC 2235, Ray Tomlinson of BBN introduced it for TENEX computers to send electronic mail over ARPANET. At that time period, there were less than 50 hosts connected to ARPANET. FTP mail and Mail Protocol were further standards that were introduced in 1973. SMTP has been in use since November 1981, when Postel published RFC 788. It became widely used in the early 1980s.

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