title: Jenkins email plugin doesn't send email to user who broke the build
body: <p>I've setup Jenkins to send emails only to users who broke the build using email-ext plugin, but I'm getting this error:</p>
  
  <blockquote>
    <p>Not sending mail to unregistered user user@example.com because your SCM claimed this was associated with a user ID ‘John Smith' which your security realm does not recognize; you may need changes in your SCM plugin</p>
  </blockquote>
  
  <p>I don't really understand what this error means, is the problem in our SCM, or in the email plugin? The emails are taken from the commit history, should I register them somewhere so Jenkins will start working?</p>
  
  <p>For reference, <a href="https://github.com/jenkinsci/email-ext-plugin/blob/master/src/main/java/hudson/plugins/emailext/plugins/recipients/RecipientProviderUtilities.java#L209" rel="noreferrer">this is the code around the error message</a> in the plugin's source code:</p>
  
  <pre><code>                    } catch (UsernameNotFoundException x) {
                          if (SEND_TO_UNKNOWN_USERS) {
                              listener.getLogger().printf("Warning: %s is not a recognized user, but sending mail anyway%n", userAddress);
                          } else {
                              listener.getLogger().printf("Not sending mail to unregistered user %s because your SCM"
                      ........
  </code></pre>
  
  <p>How do I enable <code>SEND_TO_UNKNOWN_USERS</code>?</p>
  
  <p>The error message is also mentioned in this <a href="https://issues.jenkins-ci.org/browse/JENKINS-43268" rel="noreferrer">bug report</a>.</p>