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 firstname.lastname@example.org 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
email@example.com designates xxx.xxx.xxx.xxx as
permitted sender) identity=mailfrom; client-ip=xxx.xxx.xxx.xxx;