The methodology used excludes a number of services using SparkPost. For e.g. the excerpt below of an Intercom header. It's a challenging problem to solve given the complexity of email delivery and appreciate this was a fun project to take on.
Return-Path: <bounces+XXXX@XXXX.intercom-mail.com>
Received: from mta-216-35.sparkpostmail.com (mta-216-35.sparkpostmail.com. [147.253.216.35])
by mx.google.com with ESMTPS id t13si7000210pgg.534.2020.02.03.08.37.30
for <XXXXXX>
(version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128);
Mon, 03 Feb 2020 08:37:30 -0800 (PST)
Return-Path: <bounces+XXXX@XXXX.intercom-mail.com> Received: from mta-216-35.sparkpostmail.com (mta-216-35.sparkpostmail.com. [147.253.216.35]) by mx.google.com with ESMTPS id t13si7000210pgg.534.2020.02.03.08.37.30 for <XXXXXX> (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 03 Feb 2020 08:37:30 -0800 (PST)