Internet Message Access Protocol 4 (IMAP4) is an IETF standard for the retrieval of email. IMAP4 is an alternative to POP3. In constrast to POP3, IMAP4 clients needs to stay connected to the mail server. IMAP is more oriented toward server-side operations, in contast to POP3, which features short term server connections and email download for offline processing.
IMAP4 uses the well-known TCP port 143.
See: RFC 3501.
References:
http://www.emailaddressmanager.com/tips/protocol.html
http://www.businesscreatorpro.com/articles/what-is-imap-and-pop3.php