What should I do when I get the 'MageMail API Not Found' error?

When you get this error, it usually means that the API Route field that MageMail is pointed to is somehow not right. If you shoot us a screenshot of the MageMail settings in your backend, that will usually be enough for us to troubleshoot the issue for you:

What you can also do, which may be faster, is that you can send us an admin login to your backend, if that's something you're comfortable with.