When an email message is sent with transport encryption, you can tell it from the “Received:” field the receiving mail relay adds to the message header.
> Received: from [...].google.com ([...]) by > [...].mail.protection.outlook.com ([...]) with Microsoft > SMTP Server (TLS) id [...] via Frontend Transport; [...] +0000
I’m wondering if this is always the case (e.g., because it is required by some sort of standard).
Here’s an excerpt from the header of a message I got recently from a customer support service.
> Received: from freshdesk.com (ec2-[...].compute-1.amazonaws.com [...]) > by ismtpd[...]iad1.sendgrid.net (SG) with ESMTP id [...] > for <[...]>; [...] +0000 (UTC)
And here’s another, perhaps more worrying, from ACM’s email forwarder.
> Received: from in-007.lax.mailroute.net > by acmsmtp01.acm.org (ACM Email Forwarding Service) with ESMTP id [...] > for <[...]>; [...] -0500
Can I be certain in these cases that the messages were transmitted in clear text?