Please note: MailGun often requires to contact support to enable your account - a measure they have in place against abuse
Go to https://app.mailgun.com/app/domains/new and enter your domain, ideally a subdomain, e.g.
Then follow the steps provided to verify and configure your domain
MAILGUN_PUBLIC_KEY (from https://app.mailgun.com/app/account/security)
See also Heroku