If you're experiencing an error when setting up Message Injection, here are some steps you can try:
This error may be caused by trying to test Message Injection with an email address that is not listed on the tenant. When testing the set up of Message Injection, by default the test email is sent to the email address you used to sign in to the usecure platform. Make sure you amend the email address to an email that has an inbox and that is listed in the tenant you have set up to use Message Injection.