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Post Office Protocol version 3, SMTP
The Post Office Protocol version 3 (POP3) is an application-layer Internet protocol used by local e-mail clients. Its purpose is to retrieve e-mail from a remote server over a TCP/IP connection. POP3 and IMAP4 protocols (Internet Message Access Protocol) are the two most prevalent Internet standard protocols for e-mail retrieval.
The POP3 structure and its procedures support end-users with not permanent connections to allow retrieving e-mail 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, e-mail 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.
UIDL (Unique IDentification Listing)
Clients who use leave mail on server option generally use the POP3 UIDL command. POP3 commands identify specific messages by their ordinal number on the mail server. This is a problem for a client intending to leave messages on the server, since these message numbers may change from one connection to the server to another. UIDL provides a mechanism to avoid these numbering issues. The server assigns a string of characters as a permanent and unique ID for the message.
When a POP3-compatible e-mail client connects to the server, it can use the UIDL command to get the current mapping from these message IDs to the ordinal message numbers. The client can then use this mapping to determine which messages it has yet to download, which saves time when downloading.
SMTP (Simple Mail Transfer Protocol)
Whether using POP3 or IMAP to retrieve messages, e-mail clients typically use the SMTP_Submit profile of the SMTP to send messages. E-mail clients are commonly categorized as either POP or IMAP clients, but in both cases the clients also use SMTP. There are extensions to POP3 that allow some clients to transmit outbound mail via POP3 - these are known as "XTND XMIT" extensions. The Qualcomm qpopper and CommuniGate Pro servers and Eudora clients are examples of systems that optionally utilize the XTND XMIT methods of authenticated client-to-server e-mail transmission.
Like many other older Internet protocols, POP3 originally supported only an unencrypted login mechanism. Although plain text transmission of passwords in POP3 still commonly occurs, POP3 currently supports several authentication methods to provide varying levels of protection against illegitimate access to a user's e-mail. One such method, APOP, uses the MD5 hash function in an attempt to avoid replay attacks and disclosure of the shared secret. Clients implementing APOP include Mozilla Thunderbird, Opera, Eudora, KMail, Novell Evolution, Windows Live Mail, PowerMail, and Mutt. POP3 clients can also support SASL authentication methods via the AUTH extension. MIT Project Athena also produced a Kerberized version.
POP3 works over a TCP/IP connection using TCP on network port 110. E-mail clients can encrypt POP3 traffic using TLS or SSL. A TLS or SSL connection is negotiated using the STLS command. Some clients and servers, like Google Gmail, instead use the deprecated alternate-port method, which uses TCP port 995 (POP3S).
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