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"**

This is by design when using Sitecore's Email Cloud Delivery Service through Sparkpost.

How Bounces (aka Delivery Status Notifications) Work in EXM

Email Cloud Delivery Service

The bounce@spe.sitecoremail.com email address is used as the X-Sender header so that Sparkpost knows where to email Bounces and Spam Complaints to. How that process works, is that when an email bounces, Sparkpost will forward the email, along with additional headers to the bounce@spe.sitecoremail.com email address.

This is a POP3 account that Sitecore manages. When Email Experience Manager is configured to use EmailCloud as the eds:define variable in the web.config, this activates an Agent that runs every so often, which connects to the POP3 account and retrieves all messages from the server that were sent from the EXM Manager Root.

From there, Sitecore processes the email from the POP3 and adds analytical information for Bounces and Spam Complaints. This is not a process that you can adjust.

Advice and Summary

My advice is that if you are looking at the X-Sender header for emails going through, you'll want to adjust your whitelisted email addresses and add that email to your list.

Custom SMTP Configuration

Bounces work differently in a Custom SMTP configuration. Out of the Box, Bounces are not handled without some setup and configuration. You'll need to bring your own POP3 mailbox and email address and config bounce by patching the Sitecore.EDS.Providers.CustomSmtp.Sync.config configuration.

Additionally, there are fields in the Email Manager Root for configuring the POP3 as shown in the image below.