In computing, local e-mail clients use the Post Office Protocol version 3 (POP3), an application-layer Internet standard protocol, to retrieve e-mail from a remote server over a TCP/IP connection. POP3 and IMAP4 (Internet Message Access Protocol) are the two most prevalent Internet standard protocols for e-mail retrieval. Virtually all modern e-mail clients and servers support both.

Post Office Protocol (POP) in Detail:-
The overall communication model used for TCP/IP e-mail gives many options to an e-mail user for accessing his or her electronic mailbox. The most popular access method today is the simple offline access model, where a client device accesses a server, retrieves mail and deletes it from the server. The Post Office Protocol (POP) was designed for quick, simple and efficient mail access; it is used by millions of people to access billions of e-mail messages every day.

The Post Office Protocol (POP) is currently the most popular TCP/IP e-mail access and retrieval protocol. It implements the offline access model, allowing users to retrieve mail from their SMTP server and use it on their local client computers. It is specifically designed to be a very simple protocol and has only small number of commands. The current revision of POP is version 3, and the protocol is usually abbreviated POP3 for that reason

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Internet Mail Access Protocol in Detail:-
IMAP stands for Internet Message Access Protocol. It is a method of accessing electronic mail or bulletin board messages that are kept on a (possibly shared) mail server. In other words, it permits a “client” email program to access remote message stores as if they were local. For example, email stored on an IMAP server can be manipulated from a desktop computer at home, a workstation at the office, and a notebook computer while traveling, without the need to transfer messages or files back and forth between these computers.

IMAP’s ability to access messages (both new and saved) from more than one computer has become extremely important as reliance on electronic messaging and use of multiple computers increase, but this functionality cannot be taken for granted: the widely used Post Office Protocol (POP) works best when one has only a single computer, since it was designed to support “offline” message access, wherein messages are downloaded and then deleted from the mail server. This mode of access is not compatible with access from multiple computers since it tends to sprinkle messages across all of the computers used for mail access. Thus, unless all of those machines share a common file system, the offline mode of access that POP was designed to support effectively ties the user to one computer for message storage and manipulation

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