If they signed up and used Mailgun sub domain we just ask to create a domain and set up DNS records, this usually ends all conversations. However in some cases

If they signed up and used Mailgun sub domain we just ask to create a domain and set up DNS records, this usually ends all conversations. However in some cases it gets very tricky - they can control the domain or can build an website with some non-working signup forms and services. We proceed and launch investigation up to signing up for their service - usually sign up does not work :-)