title: 5.7.1 Sender address rejected: not owned by auth user. .net core yandex mail
body: <pre><code>            SmtpClient client = new SmtpClient(&quot;smtp.yandex.com&quot;, 587);
              client.Credentials = new NetworkCredential(&quot;site@example.com&quot;, &quot;password&quot;);
              client.EnableSsl = true;
  </code></pre>
  <p>can you help me please ?</p>
  <p>An unhandled exception occurred while processing the request.
  SmtpException: Mailbox name not allowed. The server response was: 5.7.1 Sender address rejected: not owned by auth user.</p>