Em main.cf
smtpd_recipient_restrictions =
permit_mynetworks,
permit_sasl_authenticated,
reject_unauth_destination,
reject_non_fqdn_sender,
reject_non_fqdn_recipient,
reject_unknown_recipient_domain,
reject_unknown_reverse_client_hostname,
reject_unknown_client_hostname,
check_client_access hash:/etc/postfix/whitelist,
reject
Em /etc/postfix/whitelist
my.whitelisted.tld OK
Altere my.whitelisted.tld
para o domínio que você deseja permitir.
execute postmap /etc/postfix/whitelist
e service postfix restart