From the orginal IMAP.org website:

IMAP was originally developed in 1986 at Stanford University. However, it did not command the attention of mainstream email vendors until a decade later, and it is still not as well-known as earlier and less-capable alternatives such as POP, though that is rapidly changing, as articles in the trade press and the implementation of IMAP in more and more software products show. (See IMAP Status and History for a chronological overview of significant IMAP developments.)

There was a companion protocol to IMAP, developed at Carnegie Mellon University. It is called the “Application Configuration Access Protocol”, or ACAP, and provides the same location independent access to configuration files, address books, bookmark lists, etc, that IMAP offers for mailboxes.

IMAP Protocol Specification

IMAP was originally defined in RFC 2822 which has been supersceded by RFC 5322