Internet Message Access Protocol (IMAP) is an Internet standard protocol used by email clients to retrieve email messages from a mail server over a TCP/IP connection. IMAP is defined by RFC 3501. IMAP is a newer alternative to POP3.
Unlike POP3, which only downloads the messages, IMAP synchronizes them with the email server and tracks changes in their status. Another feature of IMAP is that it allows messages to be organized by folders and the email client setup to use IMAP access will also synchronize the folders beside the emails themselves.
IMAP was designed with the goal of permitting complete management of an email box by multiple email clients, therefore clients generally leave messages on the server until the user explicitly deletes them. An IMAP server typically listens on port number 143. IMAP over SSL (IMAPS) is assigned the port number 993.
Virtually all modern e-mail clients and servers support IMAP, which along with the earlier POP3 (Post Office Protocol) are the two most prevalent standard protocols for email retrieval. Many webmail service providers such as Gmail, Outlook.com and Yahoo! Mail also provide support for both IMAP and POP3.