Internet Message Access Protocol
The Internet Message Access Protocol (IMAP) is another method of reading email kept on a mail server by permitting a client to access remote message stores as if they are local. Messages stored on an IMAP server can be manipulated from multiple computers without transferring messages between the different machines. In other words, a telecommuter with a laptop computer at home and a desktop computer at the office need not worry about synchronizing each computer since mail read at either machine continues to be maintained on the server. This “online” message access contrasts with the “offline” Post Office Protocol (POP), which works best when you only have a single computer; mail messages are downloaded to the POP client and then deleted from the mail server.
While IMAP has more commands than POP, making it arguably more difficult to implement and use, the existence of development libraries and extensions give IMAP much flexibility and room for growth. Many management functions, sorting capabilities, and editing tools can be invoked on the server to improve performance and organization. Many POP clients now have similar options, but POP’s main advantage is the maturity of applications using the protocol, with a wide array of products already available.
The current version is IMAP4, defined in RFC 2060.