title: Jenkins "Not sending mail to unregistered user"
body: <p>Jenkins refuses to send error mails to some users.
  In the log I found these messages:</p>
  
  <pre><code>....
  Build step 'Maven Goals aufrufen' marked build as failure
  Not sending mail to unregistered user user1@example.org
  Sending e-mails to: user2@example.org
  Finished: FAILURE
  </code></pre>
  
  <p>The user (user1) is listed in the admin people list. He also has an registered email (user1@example.org).</p>
  
  <p><a href="https://i.stack.imgur.com/UGLZS.png" rel="noreferrer"><img src="https://i.stack.imgur.com/UGLZS.png" alt="Jenkins Peoaple admin interface"></a></p>
  
  <p>So why is this user called an unregistered user?</p>
  
  <p>When searching for this warning, I find several open JIRA issues such as <a href="https://issues.jenkins-ci.org/browse/JENKINS-43178" rel="noreferrer">https://issues.jenkins-ci.org/browse/JENKINS-43178</a>
  But these doesn't help me.</p>
  
  <p>Is this a misconfiguration or a jenkins bug?</p>