title: Anymail Mailgun Error
body: <p>I am trying to integrate django-anymail with mailgun. When I send a mail with django send_mail as</p>
  
  <pre><code>send_mail('Testing', 'Test dsadsadsa asdsa body', 'xyz@mailinator.com', ['xxxx@gmail.com'])
  </code></pre>
  
  <p>It gives an error:</p>
  
  <pre><code>AnymailRequestsAPIError: Sending a message to xxxx@gmail.com from xyz@mailinator.com
  ESP API response 404:
  {
    "message": "Domain not found: mailinator.com"
  }
  </code></pre>
  
  <p>How can I get rid of this?</p>
  
  <p>My settings file contains this:</p>
  
  <pre><code>ANYMAIL = {
      "MAILGUN_API_KEY": "key-xxxmyprivatekey",
  }
  EMAIL_BACKEND = "anymail.backends.mailgun.MailgunBackend"
  DEFAULT_FROM_EMAIL = "no-reply@mailinator.com"
  </code></pre>