Somewhere along the way I would expect an error message.
First a quick thing to look at. Sometimes you may see something interesting upon checking the following path:
Dashboard > Statistics > Settings > Notifications > Statistical report
Then, three things to consider:
You can check the email server for an error
Or Matomo may have generated errors
Or the receipient’s mail server may have stripped images
That last one happens a lot in our inbox for emails. Both the sender and receiver have email servers that do random things with inline images. Can you send through a variety of routes, a variety of recipient email servers (send it to your own personal email, and to someone else’s personal email, to test it)?