FreeSource : Документация/Postfix/BackupMX/recipientverification/rejectunverifiedrecipient

reject_unverified_recipient на backup MX в качестве средства верификации получателей

main.cf:
smtpd_restriction_classes = check_relay_recipient check_relay_recipient = reject_unverified_recipient, permit smtpd_recipient_restrictions = ... cdb:/etc/postfix/relay_domains, ...

/etc/postfix/relay_domains:
mydomain1.tld check_relay_recipient mydomain2.tld check_relay_recipient