在postfix的main.cf中設定這項，但你必須知道是否所有unknown都是不重要的信，不然擋了客戶的信又不知道為什麼就慘了！

smtpd_recipient_restrictions

These destinations do not need to be listed in $relay_domains.

The following restrictions are available (* is part of default setting):

permit_mynetworks: permit if the client address matches $mynetworks.
*reject_unknown_client: reject the request if the client hostname is unknown.*
reject_maps_rbl: reject if the client is listed under $maps_rbl_domains.
reject_invalid_hostname: reject HELO hostname with bad syntax.
reject_unknown_hostname: reject HELO hostname without DNS A or MX record.
reject_unknown_sender_domain: reject sender domain without A or MX record.