RFC-1939 ● Post Office Protocol - Version 3

USER name

Arguments:

a string identifying a mailbox (required), which is ofsignificance ONLY to the server

Restrictions:

may only be given in the AUTHORIZATION state after the POP3greeting or after an unsuccessful USER or PASS command

Discussion:

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.

The server may return a positive response even though no such mailbox exists. The server may return a negative response if mailbox exists, but does not permit plaintext