D.3.1 POP3 Command/Response Syntax

POP3 uses a command/response syntax similar to that used by SMTP . A handful of commands exist for user to login, check for mail, download messages, etc. Rather than use numeric response codes, POP3 uses "+" and "-" to indicate success or failure.

POP3 is a user-specific mail system, instead of a host-specific mail system like SMTP. POP3 commands focus on the user's mailbox, and not the entire postoffice.

Some of the most common POP3 commands are those listed in Figure D.7 below:

Figure D.7 Common POP3 commands used during POP3 mail sessions.
Command	Description
USER	User account for logging in to check/retrieve messages.
PASS	Password for above username.
STAT/LIST	List number and size of messages waiting.
RETR	Retrieve a specific message from the mailbox.
DELE	Delete a specific message from the mailbox.
QUIT	Logout and close the connection.

Rather than use the numeric response codes found in SMTP, POP3 relies on two simple status messages: "+" indicates that the command was accepted and executed properly, while "-" is used to indicate that the command was not understood or was not executed. As with SMTP, POP3 servers are allowed to put whatever text messages they want to after the "+" or "-". The POP3 client must wait for a response before issuing any more commands.

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