SMTP server response on unknown client reject
unknown_client_reject_code
This parameter specifies the SMTP server response code when a client without address
to name mapping violates the reject_unknown_clients restriction rule that is:
Access denied: Unknown SMTP client hostname (without DNS A or MX record)
Note that the postfix default value used for unknown_client_reject_code is
code 450 also used for other errors. This code in 'standard RFC 822' means
'Requested mail action not taken: mailbox busy or access denied' and
ask the sender to try again later (hoping the DNS will answer correctly
later).
Do not change this unless you have a complete understanding of RFC 822.