Is it possible to determine whether an email I received was sent over TLS?
Solution 1:
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.