The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Key Applications and Application Protocols            9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)                 9  TCP/IP Electronic Mail System: Concepts and Protocols (RFC 822, MIME, SMTP, POP3, IMAP)                      9  TCP/IP Electronic Mail Access and Retrieval Protocols and Methods                           9  TCP/IP Internet Message Access Protocol (IMAP/IMAP4)

TCP/IP Internet Message Access Protocol (IMAP/IMAP4) 1 2 3 IMAP General Operation, Client/Server Communication and Session States

IMAP History and Standards

IMAP has had a rather interesting history. I mean “interesting” in the sense that the normal orderly development process that is used for most TCP/IP protocols broke down. The result wasn't quite as bad as what occurred with SNMP, but is still unusual.

The first version of IMAP formally documented as an Internet standard was IMAP version 2 (IMAP2) in RFC 1064, published July 1988. This was updated in RFC 1176, August 1990, retaining the same version number. However, it seems that some of the people involved with IMAP were not pleased with RFC 1176, so they created a new document defining version 3 of IMAP (IMAP3): RFC 1203, published in February 1991. This is described by its authors as a “counter proposal”.

For whatever reason, however, IMAP3 was never accepted by the marketplace. Instead, people kept using IMAP2 for a while. An extension to the protocol was later created, called IMAP2bis, which added support for MIME to IMAP. This was an important development due to the usefulness of MIME, and many implementations of IMAP2bis were created. Despite this, for some reason IMAP2bis was never published as an RFC. This may have been due to the problems associated with the publishing of IMAP3.

In December 1994, IMAP version 4 (IMAP4) was published in two RFCs: RFC 1730 describing the main protocol, and RFC 1731 describing authentication mechanisms for IMAP4. IMAP4 is the current version of IMAP and the one widely used today. It continues to be refined; the latest specific version is actually called version 4rev1 (IMAP4rev1), defined in RFC 2060 and then most recently by RFC 3501. Most people still just call this “IMAP4” and that's what I will do in the rest of this section.

TCP/IP Internet Message Access Protocol (IMAP/IMAP4) 1 2 3 IMAP General Operation, Client/Server Communication and Session States