Simple Mail Transfer Protocol (SMTP) is the de facto standard for email transmission across the Internet.

SMTP is a relatively simple, text-based protocol, where one or more recipients of a message are specified (and in most cases verified to exist) and then the message text is transferred. It is quite easy to test a SMTP server using the telnet program. SMTP uses TCP port 25. To determine the SMTP server for a given domain name, the MX (Mail eXchange) DNS record is used.

SMTP started becoming widely used in the early 1980s. At the time, it was a complement to UUCP which was better suited to handle e-mail transfers between machines that were intermittently connected. SMTP, on the other hand, works best when both the sending and receiving machines are connected to the network all the time.

Sendmail was one of the first (if not the first) mail transfer agents to implement SMTP. As of 2001 there are at least 50 programs that implement SMTP as a client (sender of messages) or a server (receiver of messages). Some other popular SMTP server programs include Philip Hazel's exim, IBM's Postfix, D. J. Bernstein's Qmail, and Microsoft Exchange Server.

Since this protocol started out as purely ASCII text-based, it did not deal well with binary files. Standards such as MIME were developed to encode binary files for transfer through SMTP. Today, most SMTP servers support the 8BITMIME extension, permitting binary files to be transmitted almost as easily as plain text.

SMTP is a "push" protocol that does not allow one to "pull" messages from a remote server on demand. To do this a mail client must use POP3 or IMAP. Another SMTP server can trigger a delivery in SMTP using ETRN.

Example SMTP communication

After establishing a connection between the sender (the client) and the receiver (the server), the following is a legal SMTP session. In the following conversation, everything sent by the client is prefaced with "C:" and everything sent by the server is prefaced with "S:". On most computer systems, a connection can be established using the telnet command on the sending machine, for example

 telnet www.example.com 25 

which opens an SMTP connection from the sending machine to the host www.example.com.

 S: 220 www.example.com ESMTP Postfix C: HELO mydomain.com S: 250 Hello mydomain.com C: MAIL FROM: sender@mydomain.com S: 250 Ok C: RCPT TO: friend@example.com S: 250 Ok C: DATA S: 354 End data with <CR><LF>.<CR><LF> C: Subject: test message C: From: sender@mydomain.com C: To: friend@example.com C: C: Hello, C: This is a test. C: Goodbye. C: . S: 250 Ok: queued as 12345 C: quit S: 221 Bye 

Although optional and not shown above, nearly all clients ask the server which SMTP extensions the server supports by using the EHLO greeting, and use HELO only if the server does not respond to EHLO.

SMTP security and spamming

One of the limitations of the original SMTP is that it has no facility for authentication of senders. Therefore the SMTP-AUTH extension was defined.

In spite of this, E-mail spamming is still a major problem. Modifying SMTP extensively, or replacing it completely, is not believed to be practical, due to the network effects of the huge installed base of SMTP. Internet Mail 2000 is one such proposal for replacement.

For this reason, there are a number of proposals for sideband protocols that will assist SMTP operation. The Anti-Spam Research Group of the IRTF is working on a number of proposals for providing simple source authentication that is flexible, lightweight, and scalable. The most likely proposal to be accepted is the Sender Policy Framework (formerly known as Sender Permitted From). However, SPF is currently in jeopardy as the current specification contains sections encumbered by Microsoft patents.

See also

• DomainKeys

Related RFCs

• RFC 2821 - The Simple Mail Transfer Protocol, which recently (2001) obsoleted RFC 821
• RFC 2822 - Internet (i.e. e-mail) Message Format, which obsoletes RFC 822
• RFC 1869 - Defines the capability for SMTP service extensions, creating Extended SMTP, or ESMTP
• RFC 1891 - Delivery Status Notification (DSN) extension to SMTP