I’ve got an event setup to send the user a QR code to their email address when a token is registered for their account. I’ve had a couple users report that the email they get, the QR code is…not a QR code. Below is an example of what they get (The black box). Any ideas on how to diagnose this?
Obvious: Take a look at the source code of the email.
…and tell the users to get a real email client.
PS: It obvious: To me it looks like a qr code. Only a very down-scaled one!
What do you mean by “real” email client? This is what Outlook 365 and Exchange OWA (browser) displays. I also tried expanding the QR code object but it didn’t change it’s appearance.
I consider outlook any as a problematic email convenience software, since it does not comply to many standards.
Take a look at the source code of the email.
If this is not possible with outlook 365, you realize, why you should not use outlook 
It is the simple first step to look at the source code of the email to see, what actually should be displayed. I assume, that this is an display issue. And many outlook incarnations are known for having display issues, since they do not comply to standard, but rather define their own.
So you could take a look at your email related policies or event handlers and look at your templates and configuration. But this is much hard then to guess what went wrong, without knowing, how actually the mail looks like.