I’ve just had the same problem as the OP.
In my case, my intention was to setup a Discourse instance a few days from now. So, I setup Mailgun ahead of time by getting DNS in place first. Mailgun has permanently closed my account with the same reply text as the OP, but I never reached the point of connecting a Discourse server. I wonder what sort of false positive I have flagged. It’s bad for business to deny service to new signups.