`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.