SMTP Overview, History and Standards
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The overview and history of the TCP/IP electronic mail system describes how TCP/IP evolved from its early beginnings to its current form. Since the mechanism used to deliver e-mail is such a big part of the system as a whole, any overview of the system must of necessity discuss how delivery mechanisms have changed as well. In the case of TCP/IP, I explained how the delivery of mail evolved through many forms during the 1970s as developers sought to find effective ways of communicating e-mail messages between systems. Most of these efforts involved attempts to transmit mail using existing protocols; this makes sense, since it is easier to adapt a technology than design one from scratch.
One important achievement was the publishing of the Mail Transfer Protocol (MTP), which was first defined in RFC 772 in September 1980, then updated in RFC 780 in May 1981. MTP describes a set of commands and procedures by which two devices can connect using TCP to exchange e-mail messages. Its operation is described largely using elements borrowed from two early TCP/IP application protocols that were already in use at that time: Telnet and FTP. The commands of MTP are in fact based directly on those of FTP.
There wasn't anything inherently wrong with basing e-mail delivery on something like FTP, but defining it this way made MTP somewhat of a hack. It was also restricted to the capabilities defined by FTP, a general file transfer protocol, so it was not possible to include features in the protocol that were specific to sending and receiving mail. Due to the importance of e-mail, a specific protocol designed for the purpose of delivering e-mail was warranted. This protocol was first defined in RFC 788, published in November 1981: the Simple Mail Transfer Protocol (SMTP).
The name suggests that SMTP is simpler than the non-simple MTP that it replaced. Whether this is true or not is somewhat a matter of opinion; I do note that RFC 788 is 61 pages long, while the earlier RFC 780 was only 43 pages. What SMTP definitely has over MTP is elegance; the protocol is designed specifically for the transport of electronic mail. While it retains certain similarities to FTP, it is an independent protocol running over TCP. So, from a conceptual standpoint, it can be considered simpler than MTP. In terms of mechanics, the process SMTP uses to transfer an e-mail message is indeed rather simple, especially compared to some other protocols.
RFC 788 described the operation of SMTP carrying e-mail messages corresponding to the ARPAnet text message standard as described in RFC 733. Development of both e-mail messages and the SMTP protocol continued, of course. In August 1982, a milestone in TCP/IP e-mail was achieved when RFCs 821 and 822 were published. RFC 821 revised SMTP, and became the defining standard for the protocol for the next two decades. RFC 822, its companion standard, became the standard for TCP/IP electronic mail messages carried by SMTP.
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