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Concept
Post Office Protocol
Summary
In computing, the Post Office Protocol (POP) is an application-layer Internet standard protocol used by e-mail clients to retrieve e-mail from a mail server. Today, POP version 3 (POP3) is the most commonly used version. Together with IMAP, it is one of the most common protocols for email retrieval.
The Post Office Protocol provides access via an Internet Protocol (IP) network for a user client application to a mailbox (maildrop) maintained on a mail server. The protocol supports list, retrieve and delete operations for messages. POP3 clients connect, retrieve all messages, store them on the client computer, and finally delete them from the server. This design of POP and its procedures was driven by the need of users having only temporary Internet connections, such as dial-up access, allowing these users to retrieve e-mail when connected, and subsequently to view and manipulate the retrieved messages when offline.
POP3 clients also have an option to leave mail on the server after retrieval, and in this mode of operation, clients will only download new messages which are identified by using the UIDL command (unique-id list). By contrast, the Internet Message Access Protocol (IMAP) was designed to normally leave all messages on the server to permit management with multiple client applications, and to support both connected (online) and disconnected (offline) modes of operation.
A POP3 server listens on well-known port number 110 for service requests. Encrypted communication for POP3 is either requested after protocol initiation, using the STLS command, if supported, or by POP3S, which connects to the server using Transport Layer Security (TLS) or Secure Sockets Layer (SSL) on well-known TCP port number 995.
Messages available to the client are determined when a POP3 session opens the maildrop, and are identified by message-number local to that session or, optionally, by a unique identifier assigned to the message by the POP server. This unique identifier is permanent and unique to the maildrop and allows a client to access the same message in different POP sessions.
Official source
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