Собственно, если запустить дебиановский конфигуратор почтовика (dpkg-reconfigure -p low postfix) то к тому, что он создаст достаточно добавить вот это:
smtpd_recipient_restrictions =
permit_sasl_authenticated,
permit_mynetworks,
check_client_access hash:/etc/postfix/whitelist,
check_recipient_access hash:/etc/postfix/protected_destinations,
check_recipient_access pcre:/etc/postfix/recipient_checks,
check_sender_access hash:/etc/postfix/sender_access,
permit_mx_backup,
reject_invalid_hostname,
reject_non_fqdn_hostname,
reject_non_fqdn_sender,
reject_non_fqdn_recipient,
reject_unauth_destination,
reject_unknown_sender_domain,
reject_unknown_recipient_domain,
reject_unverified_recipient,
reject
smtpd_client_restrictions =
permit_sasl_authenticated,
permit_mynetworks,
check_client_access hash:/etc/postfix/whitelist,
check_helo_access hash:/etc/postfix/whitelist,
check_helo_access pcre:/etc/postfix/helo_regexp_access,
check_client_access pcre:/etc/postfix/client_regexp_access,
reject_unknown_client,
reject_non_fqdn_hostname,
reject_rbl_client bl.spamcop.net
smtpd_sender_restrictions = permit_mynetworks,
permit_sasl_authenticated,
check_sender_access hash:/etc/postfix/whitelist,
check_sender_access hash:/etc/postfix/sender_access,
reject_unknown_address,
reject_unknown_client
За деталями — в документацию, ибо много ;)