title: powermail: custom validator without option mandatory
body: <p>I defined a custom validator which works as expected when the form field ist filled with a value. But when the form field is left empty the custom validator is not called and the field is not validated.</p>
  <p>When i additionally activate the option &quot;mandatory&quot; I could prevent that the form is sent with an empty value but this is not possible in my case because the field must not be mandatory.</p>
  <p>More precisely:</p>
  <p>Checkbox Newsletter: when not checked the field &quot;email&quot; is not needed and therefore not mandatory. When checked the field &quot;email&quot; is needed and should therefore be mandatory and validated by my custom validator which checks if the address is in the database already.</p>
  <p>My Question:</p>
  <ul>
  <li>is there an a possibility to make the &quot;mandatory&quot; validator depending on the checkbox</li>
  <li>or is there a possibility to force the custom validator when the field is empty
  ?</li>
  </ul>