Cisco ASA 5520 - mail routing

Recently I have configured Cisco ASA 5520 firewall in our network, since than we have problem with routing mail form DMZ servers to Domain mail servers (mail servers are Domino 6.5.2). Mail routing process is not smooth, mails are collect in queue and after some time (15  30 minutes) they are routed to domain mail server. All configuration settings have been copied from our old PIX firewall. On ASA we have disabled esmtp inspection to test if the problem will be solved but the problem remains. On ASA ACL we have allowed access from DMZ servers to the Domain mail servers using smtp and lotus(1352) tcp ports.
If anyone has some suggestions, thanks in advance.