title: Sandbox subdomains are for test purposes only. Please add your own domain or add the address to recipients in Account Settings
body: <p>What does the following Mailgun error response mean when one tries to send a message?</p>
  <blockquote>
  <p>Status: 400</p>
  <p>Body:   {&quot;message&quot;: &quot;Sandbox subdomains are for test purposes only. Please add your own domain or add the address to authorized recipients in Account Settings.&quot;}</p>
  </blockquote>