how to prevent mail being dropped on the server directly

[PatrickIPM]

hi,

We are running Zimbra 5.0.9 on RHEL5.

Although very few we do get some mails which are directly delivered on the server instead of going through the proper DNS MX record first.

Received: from mail.server.com ([127.0.0.1])
by localhost (mail.server.com [127.0.0.1]) (amavisd-new, port 10024)
with ESMTP id gzkDj1V0jAnY for ;
Thu, 28 Aug 2008 22:54:15 +0800 (SGT)
Received: from [41.248.160.237] (unknown [41.248.160.237])
by mail.server.com (Postfix) with ESMTP id 996A816C056
for ; Thu, 28 Aug 2008 22:54:14 +0800 (SGT)

How can we prevent that mail is being dropped directly on the server by third party senders.

thanks, Patrick