I am reviewing the settings for the mail sending service an application I'm working on uses (the service is a mailchimp acquisition called Mandrill).

I want to ascertain, when I send a test email to myself (at gmail.com) from this service, whether the traffic between mail servers uses TLS. I have access to the email that was sent to me (I have included headers below).

I plan to ask the vendor whether they support this feature, but I also wish to find out if I can verify what they tell me personally.

Delivered-To: recipient-local@gmail.com
Received: by with SMTP id k136xxxxxxxxxxxx;
        Tue, 5 Jul 2016 07:29:11 -0700 (PDT)
X-Received: by with SMTP id v11mxxxxxxxxxxxx.36.14nnnnnnnnnnn;
        Tue, 05 Jul 2016 07:29:11 -0700 (PDT)
Return-Path: <bounce-xxxxxxx@mandrillapp.com>
Received: from mail180-123.mandrillapp.com (mail180-123.mandrillapp.com. [a.b.c.d])
        by mx.google.com with ESMTPS id mmmmmmmmmmm.nnn.2016.
        for <recipient-local@gmail.com>
        (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128);
        Tue, 05 Jul 2016 07:29:11 -0700 (PDT)
Received-SPF: pass (google.com: domain of bounce-xxxxxxx@mandrillapp.com designates a.b.c.d as permitted sender) client-ip=a.b.c.d;
Authentication-Results: mx.google.com;
       dkim=pass header.i=@mail180-123.mandrillapp.com;
       dkim=pass header.i=@mandrillapp.com;
       spf=pass (google.com: domain of bounce-xxxxxxx@mandrillapp.com designates a.b.c.d as permitted sender) smtp.mailfrom=bounce-xxxxxxx@mandrillapp.com;
       dmarc=fail (p=NONE dis=NONE) header.from=gmail.com
DKIM-Signature: v=1; a=rsa-sha1; c=relaxed/relaxed; s=mandrill; d=mail180-123.mandrillapp.com;
 h=From:Sender:Subject:Reply-To:To:Message-Id:Date:MIME-Version:Content-Type; i=sender@mail180-123.mandrillapp.com;
Received: from pmta03.mandrill.prod.suw01.rsglab.com ( by mail180-123.mandrillapp.com id xxxxxxxxxxxx for <recipient-local@gmail.com>; Tue, 5 Jul 2016 14:29:11 +0000 (envelope-from <bounce-xxxxxxx@mandrillapp.com>)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=mandrillapp.com; 
 i=@mandrillapp.com; q=dns/txt; s=mandrill; t=1467728951; h=From : 
 Sender : Subject : Reply-To : To : Message-Id : Date : MIME-Version : 
 Content-Type : From : Subject : Date : X-Mandrill-User : 
 List-Unsubscribe; bh=xxxxxxxxxxxxxxxxxxxxxxxx+xxxxxxxxxxxxxxxxxx=; 
From: recipient <recipient@gmail.com>
Sender: recipient <sender@mail180-123.mandrillapp.com>
Subject: =?utf-8?Q?Subject
Return-Path: <bounce-xxxxxxx@mandrillapp.com>
Received: from [208.nnn.nnn.nnn] by mandrillapp.com id xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx; Tue, 05 Jul 2016 14:29:11 +0000
Reply-To: <recipient@gmail.com>
To: ClientSeq ClientMcD <recipient-local@gmail.com>
Message-Id: <nnnnnnnn.nnnnnnnn142911.nnnnnnnn1aadf2.nnnnnnnn@mail180-123.mandrillapp.com>
Date: Tue, 05 Jul 2016 14:29:11 +0000
MIME-Version: 1.0

The mention of TLS in the line

Received: from [...] (mail180-123.mandrillapp.com. [...])
          by mx.google.com with ESMTPS [...]
          (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128);

makes it very clear that the connection from Mandrill to GMail was indeed TLS-protected.

The same is true for the very fact that ESMTPS (secure ESMTP) was listed as transport, see RFC 3848.

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  • 3
    In this particular case, you're absolutely right. However, more generally I think it's important to note that messages may travel via multiple servers and hence have multiple Received headers; in this case, each server may view (and modify!) the message regardless of the presence of TLS on the link between. It would also be important to ensure all Received headers report the presence of TLS (though again, this only provides security between the servers, not on the servers themselves). For end-to-end protection, you'll want something like S/MIME or PGP/GPG. – Calrion Jul 6 '16 at 0:26

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