POP, IMAP, and SMTP are Internet mail protocols. They stand for Post Office Protocol (POP), Internet Message Access Protocol (IMAP) and Simplified Mail Transfer Protocol (SMTP) respectively. Generally POP or IMAP are used to receive mail. SMTP is used to send it. Most consumer eMail clients such as Eudora and Outlook support all three protocols since they must work with any server no matter which protocol(s) it uses.

SMTP was created in 1982 as a way for servers to transfer eMail messages amongst themselves. It is defined in RFCs 822 and 823. Although it was designed for servers, nothing prevents a client from using SMTP. It is fairly common practice for clients to use SMTP to handle outgoing eMail being sent to a mail server for subsequent distribution and to use IMAP or POP to get incoming messages. SMTP operates on Port 25.

IMAP dates from the mid 1980s where it originated on a DEC-20 at Stanford and was quickly ported to Unix. IMAP is defined in RFCs 1730 and 2060. Unlike POP, IMAP is designed to leave messages on the server and thus is suitable for use with Newsgroups or for use with shared mailboxes. It is said to handle multiple mailboxes better than POP. Although IMAP can be used to handle messages offline, it is really designed primarily to operate in online mode with continual access to the message material. IMAP can be used to send messages via SMTP. The default IMAP port is 143. An administrative port may sometimes be available at 665.. It is possible to send mail via IMAP, but the capability is rarely supported by servers.

POP is designed to download messages to a client where they can then be worked with offline (relative to the server). Unlike IMAP, POP removes the messages from the server. POP can be used online and can be used to send mail via SMTP as well to receive mail. POP is the simplest of the mail protocols. POP3 is defined in RFC 1725. POP operates on Port 110. It is possible to send mail via POP, but the capability is rarely supported by servers.

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