Uma forma de bloquear o envio de emails para servidores provedores de SPAM na Internet é utilizar os parâmetros abaixo no Postfix.
No Webmin, essa configuração fica em Servers -> Postfix Mail Server -> SMTP Server Options -> Restrictions on recipient addresses.
permit_mynetworks, permit_sasl_authenticated, reject_non_fqdn_hostname, reject_non_fqdn_sender, reject_non_fqdn_recipient, reject_unauth_destination, reject_unauth_pipelining, reject_invalid_hostname, reject_rbl_client bl.spamcop.net, reject_rbl_client cbl.abuseat.org, reject_rbl_client dnsbl.sorbs.net, reject_rbl_client zen.spamhaus.org