Editing Post Office Protocol (section)

==Purpose==
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.<ref name="Windows to Linux">{{cite book | last = Allen | first = David | title = Windows to Linux | publisher = Prentice Hall | year = 2004 | page =192 | isbn = 1423902459 | url = https://books.google.com/books?id=UD0h_GqgbHgC&q=network%2B+guide+to+networks}}</ref> This design of POP and its procedures was driven by the need of users having only temporary Internet connections, such as [[dial-up Internet access|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 TCP [[List of TCP and UDP port numbers#Well-known ports|well-known port number]] 110 for service requests. [[Secure communication|Encrypted communication]] for POP3 is either requested after protocol initiation, using the [[STARTTLS|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 [[Transmission Control Protocol|TCP]] [[TCP port|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 (computer science)|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. Mail is retrieved and marked for deletion by the message-number. When the client exits the session, mail marked for deletion is removed from the maildrop.