The POP mechanism is defined in a long series of documents beginning with RFC918 (October 1984, Post Office Protocol) and culminating in RFC1939 (May 1996, Post Office Protocol - Version 3). There is also an obscure companion to RFC1081, RFC1082 (November 1988, Post Office Protocol - Version 3, Extended Service Offerings), that defines a way to use POP for group discussion, in a manner analogous to the IMAP public folder. The author of RFC1082 wrote:
Since mailers and user agents first crawled out of the primordial ARPAnet, the value of discussion groups have been appreciated (though their implementation has not always been understood).
Although never widely implemented, RFC1082 is another illustration of a major theme of this book: the close but problematic relationship between Internet mail and conferencing.
IMAP has also evolved through a long series of documents, from RFC1064 (July 1988, Interactive Mail Access Protocol - Version 2) to RFC2060 (December 1996, Internet Message Access Protocol - Version 4rev1). Confusingly, the expansion of the IMAP acronym changed along the way. Originally intended mainly for online use—that is, while connected to a mail server—IMAP evolved into a superset protocol that also supports disconnected use à la POP.
Because there was some confusion about how an IMAP client locates and gains access to shared mailboxes (public folders) on an IMAP server, RFC2342 (May 1998, IMAP4 Namespace) spells out the mechanism. Said ...