Look at the email headers. Most email clients have a menu option or a command switch to display them.
They look like this and are to be read bottom-up, so the one furthest down the list is the initial sender:
If you're using a web mail service such as gmail, I don't know if that offers this feature.
Received: (qmail 12127 invoked from network); 9 Sep 2008 13:36:05 -0000
Received: from hoboi2bl5.telenet-ops.be ([188.8.131.52]) (envelope-sender <Microsoft@newsletters.microsoft.com>) by vlad.telenet-ops.be (qmail-ldap-1.03) with SMTP for <someone@somewhere>; 9 Sep 2008 13:36:05 -0000
Received: from asok.telenet-ops.be (asok.telenet-ops.be [184.108.40.206]) by hoboi2bl5.telenet-ops.be (8.13.1/8.13.1) with ESMTP id m89Da5Oa004964 for <someone@somewhere>; Tue, 9 Sep 2008 15:36:05 +0200
Received: from ladon.telenet-ops.be (ladon.telenet-ops.be [220.127.116.11]) by asok.telenet-ops.be (Postfix) with ESMTP id 03FB960009 for <someone@somewhere>; Tue, 9 Sep 2008 15:36:05 +0200 (CEST)
Received: from delivery.pens.microsoft.com (delivery.pens.microsoft.com [18.104.22.168]) by ladon.telenet-ops.be (Postfix) with ESMTP id 46686312779 for <someone@somewhere>; Tue, 9 Sep 2008 15:36:04 +0200 (CEST)
Received: from TK2MSFTDDSQ15 ([10.40.249.22]) by delivery.pens.microsoft.com with Microsoft SMTPSVC(6.0.3790.1830); Tue, 9 Sep 2008 06:36:02 -0700