F-SECURE INTERNET GATEKEEPER FOR LINUX 4.01 Administrator's Manual page 158

2 If (1) above does not apply:
① If user authentication is enabled:
1) If the user is not added:
a) The following is sent to the client: 500 Invalid Account Auth.
② If the user name contains "@" or "#":
1) The server specified by the last "@" or "#" is accessed.
③ If (2) above does not apply:
1) If the parent server is empty:
a) The following is sent to the client: 500 USER format is USER username@hostname or
username#hostname
b) The command terminates.
2) Connects to the parent server.
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