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4. The AUTHORIZATION State

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 are described in this document: the USER and PASS command combination and the APOP command. Additional authentication mechanisms are described in [RFC1734]. While there is no single authentication mechanism that is required of all POP3 servers, a POP3 server must of course support at least one authentication mechanism.

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 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 nth message in a maildrop is assigned a message-number of "n". In POP3 commands and responses, all message-numbers and message sizes are expressed in base-10 (i.e., decimal).

Here is the summary of the QUIT command in the AUTHORIZATION state:

QUIT Command​

Arguments: none

Restrictions: none

Possible Responses:

  • +OK

Examples:

C: QUIT
S: +OK dewey POP3 server signing off

Glossary:

  • AUTHORIZATION state: Initial authentication phase
  • TRANSACTION state: Main operational phase
  • UPDATE state: Final cleanup phase
  • exclusive-access lock: Prevents concurrent modifications
  • message-number: Numeric identifier for each message
  • maildrop: User's mailbox on the server
  • authentication: Identity verification process

Protocol conventions:

  • S: indicates server sends
  • C: indicates client sends
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