I need to secure my mail server

Hi guys.

I'm using postfix.

and I noticed that I received lots of spam emails that were sent from my domain (dd4bb.com) and when I checked the header of the spam mail I saw that it used my domain ip too..

and beside that if I send any mails it is delivered to spam box.

so is that some kind of mis-configured settings or security holes in my postfix/mx configurations?

as more information, the following is the header of one of the spam messages that I have received

Delivered-To: mymail@gmail.com
Received: by 10.216.71.75 with SMTP id q53cs185466wed;
        Thu, 29 Oct 2009 15:53:52 -0700 (PDT)
Received: by 10.220.89.221 with SMTP id f29mr855523vcm.61.1256856831624;
        Thu, 29 Oct 2009 15:53:51 -0700 (PDT)
Return-Path: <info@dd4bb.com>Received: from dd4bb.com ([207.192.75.32])
        by mx.google.com with ESMTP id 5si4516871vws.1.2009.10.29.15.53.51;
        Thu, 29 Oct 2009 15:53:51 -0700 (PDT)
Received-SPF: pass (google.com: domain of info@dd4bb.com designates 207.192.75.32 as permitted sender) client-ip=207.192.75.32;
Authentication-Results: mx.google.com; spf=pass (google.com: domain of info@dd4bb.com designates 207.192.75.32 as permitted sender) smtp.mail=info@dd4bb.com
Received: by dd4bb.com (Postfix)
    id F2831158035; Fri, 30 Oct 2009 00:53:50 +0200 (EET)
Delivered-To: info@dd4bb.com
Received: from [41.141.30.85] (unknown [41.250.215.191])
    by dd4bb.com (Postfix) with ESMTPS id 5DE993804B
    for <info@dd4bb.com>; Fri, 30 Oct 2009 00:53:50 +0200 (EET)
Date: Thu, 29 Oct 2009 22:53:50 -0000
To: info <info@dd4bb.com>From: Henriette Sagal</info@dd4bb.com></info@dd4bb.com></info@dd4bb.com>