Sandbox subdomains are for test purposes only. Please add your own domain or add the address to authoriz : <p>I got this error </p>

<pre><code>GuzzleHttp\Exception\ClientException: Client error: `POST https://api.mailgun.net/v3/sandbox22f9hisujrundei134m9nf84.mailgun.org/messages.mime` resulted in a `400 BAD REQUEST` response:
{
  "message": "Sandbox subdomains are for test purposes only. Please add your own domain or add the address to authoriz (truncated...)
</code></pre>

<p>when i trying unit test for function i have ?
how i can fix that ??</p>
