Email - POP3

Post Office Protocol3 is defined in RFC 1939. RFC 2249 defines extension to RFC 1939.
Default port is 110.
Request:
Commands in the POP3 consist of a case-insensitive keyword, possibly followed by one or more arguments. All commands are terminated by a CRLF pair. Keywords and arguments consist of printable ASCII characters. Keywords and arguments are each separated by a single SPACE character. Keywords are three or four characters long. Each argument may be up to 40 characters long.
Response
Responses in the POP3 consist of a status indicator and a keyword possibly followed by additional information. All responses are terminated by a CRLF pair. Responses may be up to 512 characters long, including the terminating CRLF. There are currently two status indicators: positive ("+OK") and negative ("-ERR"). Servers MUST send the "+OK" and "-ERR" in upper case.
Life Cycle
A POP3 session progresses through a number of states during its lifetime. Once the TCP connection has been opened and the POP3 server has sent the greeting, the session enters the AUTHORIZATION state. In this state, the client must identify itself to the POP3 server. Once the client has successfully done this, the server acquires resources associated with the client's maildrop, and the session enters the TRANSACTION state. In this state, the client requests actions on the part of the POP3 server. When the client has issued the QUIT command, the session enters the UPDATE state. In this state, the POP3 server releases any resources acquired during the TRANSACTION state and says goodbye. The TCP connection is then closed.
AUTHORIZATION
Once the TCP connection has been opened by a POP3 client, the POP3 server issues a one line greeting. This can be any positive response. An example might be:
S: +OK POP3 server ready
The POP3 session is now in the AUTHORIZATION state. The client must now identify and authenticate itself to the POP3 server. Two possible mechanisms for doing this are described in this document, the USER and PASS command combination and the APOP command.
TRANSACTION
Commands:

STAT

The POP3 server issues a positive response with a line containing information for the maildrop. This line is called a "drop listing" for that maildrop. The positive response consists of "+OK" followed by a single space, the number of messages in the maildrop, a single space, and the size of the maildrop in octets. This memo makes no requirement on what follows the maildrop size.
Examples:
     C: STAT
     S: +OK 2 320

LIST [msg]

Argument: a message-number (optional), which, if present, may NOT refer to a message marked as deleted. If an argument was given and the POP3 server issues a positive response with a line containing information for that message. This line is called a "scan listing" for that message.
If no argument was given and the POP3 server issues a positive response, then the response given is multi-line. After the initial +OK, for each message in the maildrop, the POP3 server responds with a line containing
information for that message. This line is also called a "scan listing" for that message.
Examples:

C: LIST
S: +OK 2 messages (320 octets)
S: 1 120
S: 2 200
S: .
...
C: LIST 2
S: +OK 2 200
... C: LIST 3
S: -ERR no such message, only 2 messages in maildrop

RETR msg

If the POP3 server issues a positive response, then the response given is multi-line. After the initial +OK, the POP3 server sends the message corresponding to the given message-number, being careful to byte-stuff the termination character (as with all multi-line responses).
Example:
Examples:
C: RETR 1
S: +OK 120 octets
S:
S: .

DELE msg

The POP3 server marks the message as deleted. Any future reference to the message-number associated with the message in a POP3 command generates an error. The POP3 server does not actually delete the message until the POP3 session enters the UPDATE state.
Examples:
C: DELE 1
S: +OK message 1 deleted
...
C: DELE 2
S: -ERR message 2 already deleted

NOOP

The POP3 server does nothing, it merely replies with a positive response.

RSET

If any messages have been marked as deleted by the POP3 server, they are unmarked. The POP3 server then replies with a positive response.

UPDATE
When the client issues the QUIT command from the TRANSACTION state, the POP3 session enters the UPDATE state. (Note that if the client issues the QUIT command from the AUTHORIZATION state, the POP3 session terminates but does NOT enter the UPDATE state.)

QUIT

The POP3 server removes all messages marked as deleted from the maildrop and replies as to the status of this operation.

Optional Commands

TOP msg n

return top n lines in mail msg.

UIDL

It is called a "unique-id listing" for that message. A unique-id listing consists of the message-number of the message, followed by a single space and the unique-id of the message. No information follows the unique-id in the unique-id listing.

USER name

To authenticate using the USER and PASS command combination, the client must first issue the USER command.

APOP name digest.

A POP3 server which implements the APOP command will include a timestamp in its banner greeting. The syntax of the timestamp corresponds to the `msg-id' in [RFC822], and MUST be different each time the POP3 server issues a banner greeting.