Topic: Postfix not resolving server IP in headers

I noticed that I was getting a spam test resulting in positive, albeit a very very low score [like 0.001], triggering because the sending server IP supposedly had no reverse mapping (PTR record, that is, a FQDN with A and PTR). The server does indeed have a PTR record and resolves fine. This happened with several other mails I received; in fact, not one single incoming mail had the source server IP reversing. Nameservers on the server in resolv.conf are configured correctly.

See the excerpt from the header below:

Received: from mail-qy0-f187.google.com (unknown [])
    by mail.mydomain.com (Postman) with ESMTP id 3E538140293
    for <user@mydomain.com>; Wed, 28 Apr 2010 14:01:22 +0400 (MSD)

The source server IP is and postfix is reporting 'unknown' when it should be mail-qy0-f187.google.com. Is there a configuration option in postfix that is disabling this lookup? Perhaps for speed purposes or something? I cannot find anything about it.