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Simple Mail Transfer Protocol (SMTP) is the de facto standard for e-mail transmissions across the Internet. Formally SMTP is defined in RFC 821 (STD 10) as amended by RFC 1123 (STD 3) chapter 5. The protocol used today is also known as ESMTP and defined in RFC 2821.

History

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 an SMTP server using the telnet program (see below).

SMTP uses TCP port 25. To determine the SMTP server for a given domain name, the MX (Mail eXchange) DNS record is used, falling back to a simple A record in the case of no MX (not all MTAs (Mail Transfer Agents) support fallback).

SMTP started becoming widely used in the early 1980s. At the time, it was a complement to UUCP (Unix to Unix CoPy) 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.

The article about sender rewriting contains technical background info about the early SMTP history and source routing before RFC 1123 (1989, obsoleted by RFC 2821).

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 Postfix, qmail, Novell GroupWise, Exim, Novell NetMail 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 Multipurpose Internet Mail Extensions MIME were developed to encode binary files for transfer through SMTP. MTAs developed after sendmail also tended to be implemented 8-bit-clean, so that the alternate "just send eight" strategy could be used to transmit arbitrary data via SMTP. Non-8-bit-clean MTAs today tend to 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.

ETRN (Extended Turn) is an extension to the SMTP e-mail protocol. ETRN is defined in RFC 1985. It enables a mail server to request a second mail server to forward it outstanding mail messages. The second mail server should initiate a new SMTP connection back to the first server. Some level of security is provided by DNS-based authentication of the initiating server. The ETRN operation is useful for intermittently connected mail servers.

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. However, the impracticalities of widespread SMTP-AUTH implementation and management means that E-mail spamming is not and cannot be addressed by it.

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.

Spam is enabled by several factors, including vendors implementing broken MTAs (that do not adhere to standards, and therefore make it difficult for other MTAs to enforce standards), the insecurities of Microsoft Windows (when combined with always-on broadband connections) that allow spammers to remotely control end-user PCs and cause them to send spam, and a regrettable lack of "intelligence" in many MTAs still a major problem.