Click on the header link in the mail server queue:

Received: (qmail 30501 invoked by uid 10077); 3 Dec 2009 21:47:42 -0500
Date: 3 Dec 2009 21:47:42 -0500
Message-ID: <20091204024742.30498.qmail@hostserver.com>
To: email@domain.com
Subject: This is a test.
MIME-Version: 1.0
Content-type: text/html; charset=iso-8859-1
From: <user@domain.com>

It says its invoked by uid 10077 which corresponds to the account on the /etc/password file:

-bash-3.2# more /etc/passwd | grep 10077
username:x:10077:2522::/var/www/vhosts/domain.com:/bin/false