As a precaution, our distribution groups have a limited number of email addresses that they will accept messages from. In anticipation of the EXM usage, the email used (e.g. SitecoreExm@domain.com) was added as a permited sender to the distribution group.
The assumption was the sending system would mark the email as coming from SitecoreExm@domain.com, however, these emails had bounce@spe.sitecoremail.com inserted into the headers. Since that was not the expected address, the email failed to reach the target audience.
Here is part of the header generated
Received: from emailserver.com (xxx.xxx.xxx.xxx) by
emailserver.com (xxx.xxx.xxx.xxx) with SMTP Server id
Fri, 8 Mar 2019 09:07:53 -0300
Received-SPF: Pass (emailserver.com: domain of
bounce@spe.sitecoremail.com designates xxx.xxx.xxx.xxx as
permitted sender) identity=mailfrom; client-ip=xxx.xxx.xxx.xxx;
receiver=emailserver.com;
envelope-from="bounce@spe.sitecoremail.com";
x-sender="bounce@spe.sitecoremail.com"**;
x-conformance=spf_only; x-record-type="v=spf1";
**x-record-text="v=spf1 exists:%{i}._spf.e.sparkpost.com
a:sitecore.mail.e.sparkpost.com ~all"**