<p>Sure. Just quote them and give the source:</p>

<blockquote>
  <p>john.doe@example.com answered my question in a private e-mail, I'm posting
  his answer below:</p>
  
  <blockquote>
    <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do
    eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad
    minim veniam, quis nostrud exercitation ullamco laboris nisi ut
    aliquip ex ea commodo consequat. Duis aute irure dolor in
    reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla
    pariatur. Excepteur sint occaecat cupidatat non proident, sunt in
    culpa qui officia deserunt mollit anim id est laborum.</p>
  </blockquote>
</blockquote>

<p>Of course assuming that the person who contacted you has nothing against quoting them publicly. If in doubt, you can always make them anonymous source, and paraphrase their answer rather then quoting it directly.</p>

<p><strong>I would encourage you to do so</strong>. We gain nothing from having unanswered questions hanging in the void, while the answer may be helpful for other users as well. Moreover, if by chance, the answer is wrong, you can get comments pointing out the mistake, so it may help you to validate it.</p>
