The POP3 reference article from the English Wikipedia on 24-Jul-2004
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POP3

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Post Office Protocol version 3 is an application layer Internet standard protocol used to retrieve email from a remote server to a local client over a TCP/IP connection. Nearly all individual Internet service provider email accounts are accessed via POP3.

The earlier versions of the POP protocol, POP (informally called POP1) and POP2, have been thoroughly made obsolete by POP3. In contemporary usage, the less precise term POP almost always means POP3 in the context of email protocols.

POP3 and its predecessors are designed to allow end users with intermittent connections such as dial-up connections to retrieve email when connected, and then to view and manipulate the retrieved messages without needing to stay connected. Although most clients have an option to leave mail on server, email clients using POP3 generally connect, retrieve all messages, store them on the user's PC as new messages, delete them from the server, and then disconnect. In contrast, the more modern IMAP email retrieval protocol supports both connected and disconnected modes of operation. Email clients using IMAP generally leave messages on the server until the user explicitly deletes them. This and other facets of IMAP operation allow multiple clients to access the same mailbox. Most email clients can be configured to use either POP3 or IMAP to retrieve messages, however ISP support for IMAP is not as common.

Whether using POP3 or IMAP to retrieve messages, clients use the SMTP protocol to send messages. Email clients are sometimes referred to as either POP or IMAP clients, but in both cases SMTP is also used.

Email attachments and non-ASCII text are nearly universally conveyed in email in accordance with MIME formatting rules. Neither POP3 nor SMTP require email to be MIME formatted, but since essentially all internet email is MIME formatted POP clients by default must also understand and use MIME. IMAP is designed to assume email is MIME formatted.

Like many other older Internet protocols, POP3 originally supported only an unencrypted login mechanism. Although plain text transmission of passwords in POP3 is still common, POP3 currently supports several authentication methods to provide varying levels of protection against illegitimate access to a user's email. It is also possible to encrypt POP3 traffic using SSL.

POP3 works over a TCP/IP connection using network port 110.

See also

POP3 RFCss