Why am I getting a bounce email about an email being removed from a thread?

I you received a bounce back email like this:

The following email(s) were removed from the thread: test@example.com. 
Please use their @user.commercehero.io email address instead.

...the reason is that you included a regular email address as a CC alongside a @user.commercehero.io email address. We don't allow non-commercehero email addresses to be CC'd as that could leak personal email addresses through the platform.