The IMAP protocol (Internet Message Access Protocol) was first conceived in 1986, at Stanford university. IMAP2 was defined in 1987, and the first Unix server was implemented at this time.
In July of 1988, the first IMAP RFC, RFC 1064 was published, and work began on the C-client. This was a series of library routines, written at Stanford by Mark Crispin. This code was originally intended as the low level foundation for a Macintosh client. Based on this was the first real IMAP client, MM-D (for "MM on Xerox D machines" -- MM was a popular DEC-20 mail program) was written in Interlisp for Xerox Lisp machines.
At that time, there was no name for the embryonic Mac client, but since it was the first one to be written in C instead of Lisp, it was given a development name of "C client". This name subsequently became "c-client" because that is the name of the subdirectory on UNIX where the source files were stored.
In 1989, Mark Crispin was hired by the University of Washington, where he continued work on the c-client. The c-client was subsequently incorporated into the University of Washington's PINE mail client, which uses fuctionality provided by c-client for RFC-822 parsing, MIME parsing and decoding and SMTP.
By 1992, the University of Washington had deployed an IMAP server, and release PINE version 2.0, with IMAP support. Carnegie Mellon University began work on the Andrew II/Cyrus project, a competing offering
With the release of the RFC's for IMAP4 (1730-1733), IMAP4 was appropved as a proposed Internet standard, and the pace of development accelerated. By 1995 Carnegie Mellon had released its first IMAP4 server, and an IMAP4 server and rewritten c-client had been released by the University of Washington. With the release of the IMAP4rev1 specification (RFC 2060) in 1996, and the declaration of support for IMAP by Sun and Netscape, the way was paved for IMAP to become ubiquitous.