There are three versions of POP (Post Office Protocol): POP, POP2, and POP3. The detailed specification of POP is described in RFC 918 (obsolete), RFC 937, RFC 1081, and RFC 1082.
Basically, the intent of the original Post Office Protocol (POP) was to let a user's workstation access mail from a mailbox server. It uses the Simple Mail Transfer Protocol (SMTP) to post mail from the workstation to the mailbox server. The POP protocol is dependent on TCP (Transmission Control Protocol). Below is the POP protocol:
The server is listening for a connection with the protocol. When a connection is opened, the server sends a greeting message and waits for commands. When commands are received, the server acts on them and responds with replies.
The client opens a connection, waits for the greeting, then sends the USER and then the PASS commands to establish authorization to access mailboxes. The client begins a mail reading transaction with either an RDEL (to read and delete all messages from a mailbox) or a RETR (to simply read all messages from a mailbox). The server opens and locks the mailbox and responds with the number of characters in the mailbox. Then, the client asks for the data to be sent by issuing the RCEV command. The server responds by sending the mail data.
When all the data has been received, the client sends the RCVD command. If the transaction started with the RDEL command, the server now deletes the mail data from the mailbox. In any case, the server closes and unlocks the mailbox. The client terminates the session with the QUIT command. POP2 is designed for an environment of workstations and servers on a low-delay, high-throughput, local networks (like Ethernets). The POP2 server is listening on TCP port 109.
POP3 lets a workstation dynamically access a maildrop on a server host. Usually, this means the POP3 is the means a workstation (client) uses to retrieve mail from the server. The POP3 server listens on TCP port 110.
Any UNIX system (like A/UX) that supports TCP can implement a POP server. There is public domain POP server software available from UC Berkeley at the following Internet address (via ftp "anonymous" and any password):
lilac.berkeley.edu 128.32.136.12
or
jagubox.gsfc.nasa.gov
Article Change History:
12 Sep 1994 - Reviewed. Added info on jagubox.
31 Aug 1992 - Reivewed.
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