Postfix amavis headers

Hello,

can someone tell me how to change 'unknown' in the headers of my mail.

ex:

Received: from localhost (unknown [127.0.0.1]) by
zeus.vortex.dnsalias.org
(Postfix) with ESMTP id AFA7233E8B for
<diego.claeys@vortex.dnsalias.org>;
Wed, 26 Dec 2007 13:59:25 +0100 (CET)
Received: from zeus.vortex.dnsalias.org ([127.0.0.1]) by localhost
(zeus.vortex.dnsalias.org [127.0.0.1]) (amavisd-new, port
10024) with ESMTP
id 16281-04 for <diego.claeys@vortex.dnsalias.org>; Wed, 26
Dec 2007
13:59:16 +0100 (CET)

Thanks.