Taken from rfc1939.txt.
Minimal POP3 Commands:

    USER name               valid in the AUTHORIZATION state
    PASS string
    QUIT

    STAT                    valid in the TRANSACTION state
    LIST [msg]
    RETR msg
    DELE msg
    NOOP
    RSET
    QUIT

Optional POP3 Commands:

    APOP name digest        valid in the AUTHORIZATION state
    TOP msg n               valid in the TRANSACTION state
    UIDL [msg]

POP3 Replies:

    +OK
    -ERR

Example

    C: USER mrose
    S: +OK mrose is a real hoopy frood
    C: PASS secret
    S: +OK mrose's maildrop has 2 messages (320 octets)