POP3 has many commands that it can interpret but what is the significance of the USER command?
When a mail client first connects to a POP3 mail server, the server will
respond with a welcome message e.g.
+OK POP3 server ready
point, the mail client needs to indicate who it is representing. It does
this by sending the "USER" command. The command must be followed
by one piece of information – the name of the user. Here are several
examples of the USER command:
Each one of this commands tells the server who the
user is. All secure servers (like GMS) will always respond with an
"OK" response and await a password (with the "PASS"
command) before allowing access to email. The server always responds OK to
the "USER" command so that someone attempting to attack the
system can’t find out of the user account exists without completeing the
log on with a valid password.