Post Office Protocol version 3 (POP3)

We refer you to the documents linked below for complete details, but quote here that the official POP3 specification describes POP3 as follows:

"The Post Office Protocol - Version 3 (POP3) is intended to permit a workstation to dynamically access a maildrop on a server host" ... "Usually, this means that the POP3 protocol is used to allow a workstation to retrieve mail that the server is holding for it."

Put more simply, a POP3 Server stores e-mail for clients and delivers that e-mail to those clients over the network when they ask for it. Furthermore, the POP3 protocol provides a "lock-step" transactional session which must be successfully completed before any messages can be deleted from the server. If a POP3 session fails for any reason, the original message(s) are retained on the server.

Note: a POP3 Server does not "exchange" mail for clients with remote servers. In other words, it does not act as a MX (Mail Exchanger) in the DNS (Domain Name System) for purposes of sending e-mail--that is the function of a SMTP (Simple Mail Transfer Protocol) Server.

The Post Office Protocol version 3 (POP3), designated as STD 53 of the "Internet Official Protocol Standards," was detailed in Internet "Request For Comments" document RFC 1939. Although the POP3 AUTHentication Command (introduced in RFC 1734 to enhance the security of the protocol after an earlier draft of the POP3 specification (RFC 1725)), was referenced as a footnote in RFC 1939, it was not formally included in the final POP3 specification. Consequently, a POP3 Extension Mechanism was adopted in RFC 2449, which formally added the AUTH command (and others) to the protocol and provided both new functionality (capabilities responses and error responses) and flexibility for future extensions to the POP3 protocol. A recent example of the use of the POP3 Extension Mechanism was in RFC 3206, which added new error response codes to the protocol.

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