/etc/postfix/main.cf:
    smtpd_recipient_restrictions =
        check_recipient_access hash:/etc/postfix/protected_destinations

    smtpd_restriction_classes = insiders_only
    insiders_only = check_sender_access hash:/etc/postfix/insiders, reject

/etc/postfix/protected_destinations:
    domain.tld   insiders_only

/etc/postfix/insiders:
    1.2.3.4         OK  this address is allowed to send mail to domain1.tld
    this.host.tld   OK  this address is allowed to send mail to domain1.tld