Hello,
dbowdley.
Most SMTP servers perform reverse DNS (RDNS) checks and include the domain name in the "HELO" as well as the domain name disclosed by the RDNS check in the "Received" header.
Here is a redacted example of a Fastmail Received header with RDNS check:
Code:
Received: from mail.dovecot.fi (wursti.dovecot.fi [94.237.32.243])
by mx5.messagingengine.com (Postfix) with ESMTP
for <[USERNAME]@fastmail.fm>; Sun, 22 Jul 2018 10:36:09 -0400 (EDT)
Here is a redacted example of a Runbox Received header (apparently Runbox doesn't perform a RDNS check or doesn't include the RDNS domain in the Received header):
Code:
Received: from mail-it0-f44.google.com ([209.85.214.44])
by mailfront10.runbox.com with esmtps (TLS1.2:RSA_AES_128_CBC_SHA1:128)
(Exim 4.82)
id 1fhcDx-0001Ah-VX
for [USERNAME]@runbox.eu; Mon, 23 Jul 2018 16:54:10 +0200
As a practical matter, though, most people are unaware of Received headers. Even people who know about them don't bother looking at them.
--
Jacinto