We got this answer:
I’m not sure I understand what you’re looking to do. Currently, on your account, you just have the sandbox domain that is included and you’re only allowed 5 Authorized recipients. So if you want to add more to that, you have to remove other recipients. Alternatively, if you own a domain, you can verify your domain through Mailgun so you can send messages using your custom domain. Below I’ve linked our documentation on adding your domain to Mailgun.
What it says is totally different what you say.