9. POP3 Command Summary

Minimal POP3 Commands:

Command	Valid States	Arguments	Description
USER	AUTHORIZATION	username	User identification
PASS	AUTHORIZATION	password	Password
QUIT	Any state	none	End session
STAT	TRANSACTION	none	Get mailbox statistics
LIST	TRANSACTION	[msg]	List messages
RETR	TRANSACTION	msg	Retrieve message
DELE	TRANSACTION	msg	Delete message
NOOP	TRANSACTION	none	No operation
RSET	TRANSACTION	none	Reset deleted marks

Optional POP3 Commands:

Command	Valid States	Arguments	Description
APOP	AUTHORIZATION	name digest	Authenticated login
TOP	TRANSACTION	msg n	Get message headers + n lines
UIDL	TRANSACTION	[msg]	Get unique message ID

POP3 Replies:

Response	Meaning
+OK	Success
-ERR	Failure

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State Transitions:

[Initial Connection]
        ↓
  AUTHORIZATION
        ↓ (successful authentication)
  TRANSACTION
        ↓ (QUIT command)
    UPDATE
        ↓
  [Connection closed]