503 reply codes
503 : Authentication already established
503: Command command rejected - command not allowed after prior_command command
503: command terminated due to server shutdown in progress
503: EPSV ALL received - command not allowed after EPSV ALL
503 : Must first establish authentication
503 : Must first issue PBSZ
503 : Must issue AUTH command before ADAT command
503: PASS command must be preceded by USER
503 : Server on the TLS secure port does not accept the AUTH command
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FTPD reply codes