implement a test api key
In Mailgun you should be able to use a test api key to experiment with Mailgun's API. No mail should be sent, but webhooks should trigger normally and you should be able to generate synthetic bounces and complaints without impacting your reputation.
This also lets the developer to setup a development environment that is similar to the production environment without the risk of sending any mails by misstake.
Charlie Hayes commented
I would be happy with a separate API key to use with the sandbox domain.
Definitely Something useful. Granted we can redirect to the spam folder the mails, but that's a lot of bother. please implement test key so that we can sell your services to our client during demo more easily, please. Makes everyone look better :)
That is needed right for security reasons.