SMTP Overview, History and Standards
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SMTP Extensions and Revisions
As the 1980s progressed and TCP/IP and the Internet both grew in popularity, SMTP gradually overtook other methods to become the dominant method of e-mail message delivery. For a number of years, the protocol was used mostly as is, with no new RFCs published to define new versions or formally change its behavior.
This changed in February 1993, when RFC 1425, SMTP Service Extensions, was published. As the name suggests, this standard describes a process for adding new capabilities to extend how SMTP works, while maintaining backward-compatibility with existing systems. SMTP with these extensions is sometimes called Extended SMTP or ESMTP (though use of this term seems to be not entirely universal). As development of SMTP continued, RFC 1425 was revised in RFC 1651 in July 1994 and then RFC 1869 in November 1995. Along with these, a number of other RFCs defining particular SMTP extensions such as pipelining and message size declaration were defined.
In April 2001, another major milestone in TCP/IP e-mail was reached when revisions of RFC 821 and RFC 822 were published, as RFCs 2821 and 2822 respectively. Both documents are consolidations of updates and changes that had been made to RFCs 821 and 822 between 1982 and 2001. And no, I don't think it is a coincidence that the old and new RFC numbers are exactly 2000 apart. RFCs 2820 and 2823 were both published in May 2000, so it looks like 2821 and 2822 were reserved for the e-mail standards. I think this was a great idea, as it makes more clear that the new RFCs are revisions of the old ones.
RFC 2821 is the current base standard for SMTP. It incorporates the base protocol description from RFC 821, and the latest SMTP extensions as defined in RFC 1869. Perhaps more importantly, it updates the description of the e-mail communication model to reflect the realities of modern TCP/IP networks, especially the e-mail features built into the Domain Name System (DNS). We'll examine this in more detail in the next topic.
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