Here's the full "original" details for one of the most recent transactional emails, if this helps (with my personal addresses removed). My host is TMDhosting and the sending program is PHP Mail.


Delivered-To: [myemail]
Received: by 10.216.161.130 with SMTP id w2csp140971wek;
Wed, 11 Jul 2012 15:03:23 -0700 (PDT)
Received: by 10.68.193.195 with SMTP id hq3mr81612704pbc.30.1342044203069;
Wed, 11 Jul 2012 15:03:23 -0700 (PDT)
Return-Path: <SRS0=2Bx9=FM=disruptivate.biz=admin@bounce.secureserver.net>
Received: from p3plsmtp02-01.prod.phx3.secureserver.net (p3plsmtp02-01.prod.phx3.secureserver.net. [72.167.218.31])
by mx.google.com with ESMTP id vh4si5836115pbc.353.2012.07.11.15.03.22;
Wed, 11 Jul 2012 15:03:22 -0700 (PDT)
Received-SPF: pass (google.com: domain of SRS0=2Bx9=FM=disruptivate.biz=admin@bounce.secureserver.net designates 72.167.218.31 as permitted sender) client-ip=72.167.218.31;
Authentication-Results: mx.google.com; spf=pass (google.com: domain of SRS0=2Bx9=FM=disruptivate.biz=admin@bounce.secureserver.net designates 72.167.218.31 as permitted sender) smtp.mail=SRS0=2Bx9=FM=disruptivate.biz=admin@bounce.secureserver.net
Received: (qmail 5788 invoked from network); 11 Jul 2012 22:03:22 -0000
Delivered-To: [myemail]
Received: (qmail 5780 invoked by uid 30297); 11 Jul 2012 22:03:22 -0000
Received: from unknown (HELO p3pismtp01-025.prod.phx3.secureserver.net) ([10.6.12.30])
(envelope-sender <admin@disruptivate.biz>
by p3plsmtp02-01.prod.phx3.secureserver.net (qmail-1.03) with SMTP
for <[myemail]>; 11 Jul 2012 22:03:22 -0000
X-IronPort-Anti-Spam-Result: AqVKAIr3/U+4mjFKY2dsb2JhbABFBoI5bIonTKgdAQMBAQKBLhgXCgsIFCeCWQEVRjMBAQEEgQoEBYd8nDWGTowHjnSLQDACQAEHTw9uCAEDAoMaA4gWNYUciGWOF4NagToHgT4
Received: from node01.tmdhosting940.com ([184.154.49.74])
by p3pismtp01-025.prod.phx3.secureserver.net with ESMTP; 11 Jul 2012 15:03:21 -0700
Received: from disrupt0 by node01.tmdhosting940.com with local (Exim 4.77)
(envelope-from <admin@disruptivate.biz>
id 1Sp50C-002xRM-O8
for [myemail]; Wed, 11 Jul 2012 17:03:20 -0500
To: [myemail]
Subject: Account activated for Bill Gates at Disruptivate Goes Vertical 2012
Date: Wed, 11 Jul 2012 17:03:20 -0500
From: Disruptivate <admin@disruptivate.biz>
Reply-To: Disruptivate <admin@disruptivate.biz>
Message-ID: <720c1527deaa15a6ece9598b90ecb54c@disruptivate.biz>
X-Priority: 3
X-Mailer: PHPMailer 5.2.1 (http://code.google.com/a/apache-extras.org/p/phpmailer/)
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Content-Type: text/plain; charset="iso-8859-1"
X-AntiAbuse: This header was added to track abuse, please include it with any abuse report
X-AntiAbuse: Primary Hostname - node01.tmdhosting940.com
X-AntiAbuse: Original Domain - manyspears.com
X-AntiAbuse: Originator/Caller UID/GID - [1734 32003] / [47 12]
X-AntiAbuse: Sender Address Domain - disruptivate.biz
X-Source: /usr/bin/perl
X-Source-Args: /usr/local/bin/perl /usr/sbin/sendmail -t -i -oi -f admin@disruptivate.biz
X-Source-Dir: disruptivate.biz:/public_html

Hey Tom,
Well, if you have provided a valid API key, enabled the plugin then it should send the mails through the mandrill service.
Maybe try to make mandrill the first system plugin?
Regards,
Daniel

I figured it out. I created an API key in the backend of my existing Mailchimp account, which had an option there to create a Mandrill API key. This key didn't work. However, when I logged into Mandrill (using my mailchimp credentials) and created a key on mandrillapp.com, this new key worked great.

This second key replaced the first one within the mailchimp.com site, FYI.

So- if you are an existing mailchimp customer, be sure to create your key on mandrillapp.com!