Once the TCP connection has been opened by a POP3 client, the POP3 server issues a one line greeting. This can be any string terminated by CRLF. An example might be:
S: +OK POP3 server ready
Note that this greeting is a POP3 reply. The POP3 server should always give a positive response as the greeting.
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. The APOP command is described later in this document.
To authenticate using the USER and PASS command combination, the client must first issue the USER command. If the POP3 server responds with a positive status indicator ("+OK"), then the client may issue either the PASS command to complete the authentication, or the QUIT command to terminate the POP3 session. If the POP3 server responds with a negative status indicator ("-ERR") to the USER command, then the client may either issue a new authentication command or may issue the QUIT command.
When the client issues the PASS command, the POP3 server uses the argument pair from the USER and PASS commands to determine if the client should be given access to the appropriate maildrop.
Once the POP3 server has determined through the use of any authentication command that the client should be given access to the appropriate maildrop, the POP3 server then acquires an exclusive- access lock on the maildrop, as necessary to prevent messages from being modified or removed before the session enters the UPDATE state. If the lock is successfully acquired, the POP3 server responds with a positive status indicator. The POP3 session now enters the TRANSACTION state, with no messages marked as deleted. If the the maildrop cannot be opened for some reason (for example, a lock can not be acquired, the client is denied access to the appropriate maildrop, or the maildrop cannot be parsed), the POP3 server responds with a negative status indicator. (If a lock was acquired but the POP3 server intends to respond with a negative status indicator, the POP3 server must release the lock prior to rejecting the command.) After returning a negative status indicator, the server may close the connection. If the server does not close the connection, the client may either issue a new authentication command and start again, or the client may issue the QUIT command.
After the POP3 server has opened the maildrop, it assigns a message- number to each message, and notes the size of each message in octets. The first message in the maildrop is assigned a message-number of "1", the second is assigned "2", and so on, so that the n'th message in a maildrop is assigned a message-number of "n". In POP3 commands and responses, all message-number's and message sizes are expressed in base-10 (i.e., decimal).
Here are summaries for the three POP3 commands discussed thus far:
USER name Arguments: a string identifying a mailbox (required), which is of significance ONLY to the server Restrictions: may only be given in the AUTHORIZATION state after the POP3 greeting or after an unsuccessful USER or PASS command Possible Responses: +OK name is a valid mailbox -ERR never heard of mailbox name Examples: C: USER mrose S: +OK mrose is a real hoopy frood ... C: USER frated S: -ERR sorry, no mailbox for frated here PASS string Arguments: a server/mailbox-specific password (required) Restrictions: may only be given in the AUTHORIZATION state after a successful USER command Discussion: Since the PASS command has exactly one argument, a POP3 server may treat spaces in the argument as part of the password, instead of as argument separators. Possible Responses: +OK maildrop locked and ready -ERR invalid password -ERR unable to lock maildrop Examples: C: USER mrose S: +OK mrose is a real hoopy frood C: PASS secret S: +OK mrose's maildrop has 2 messages (320 octets) ... C: USER mrose S: +OK mrose is a real hoopy frood C: PASS secret S: -ERR maildrop already locked QUIT Arguments: none Restrictions: none Possible Responses: +OK Examples: C: QUIT S: +OK dewey POP3 server signing off