Mark Crispin made IMAP to be a present-day alternative to the prevalently used POP3 email-retrieval protocol. IMAP is an application-layer Internet protocol used for accessing email on a remote server from a local client. IMAP and POP3 are the two most widely used Internet protocols for retrieving email.
IMAP’s main advantage over POP3 is that messages can remain on the server and be accessed from more than one client (for example, a stationary office computer and a PDA) while keeping track of which messages have already been read. Both IMAP and POP3 are supported by modern email clients and servers. The present version of IMAP, known as IMAP version 4, revision 1 (IMAP4rev1), is defined by RFC 3501.
GNU_FDL. Internet Message Access Protocol. [Online, 2004.] GNU Free Documentation License Website. http://www.wordiq.com/definition/IMAP.