Sending E-Mail - Setting O/S User

Hi, When someone sends an e-mail using php, exim sets the following 2 headers (along with others of course): Received: from x.x.x.x (EHLO host.mydomain.com) (x.x.x.x) by mta151.mail.dcn.yahoo.com with SMTP; Wed, 24 Sep 2005 10:29:04 -0700 Received: from <O/S User> by host.mydomain.com with local (Exim 4.52) id 3ABiAO-00019o-KL for yahoouser@yahoo.com; Thu, 27 Oct 2005 13:29:03 +0300 Here, the <O/S User> is picked up from the operation system, and it is the user that owns the process, which is the process that initiates sending out this e-mail. Is there a way to specify a certain user instead of the owner of the current process, or is there a way to configure exim such that, it won't pick up the owner of that process for this header, but a specific user that I explicitly specify through configuration? Thanks...