title: Removing 'on behalf of' when sending mail using MailGun
body: <p>I'm trying to figure out how to send mails using the MailGun Golang API without having it sent 'on behalf of'.  </p>
  
  <p>This is how the From address currently looks (where foo.com is the sender's email domain and bar.com is my domain):</p>
  
  <pre><code>john=foo.com@mail.bar.com on behalf of John Smith &lt;john@foo.com&gt;
  </code></pre>
  
  <p>What do I need to do so that it looks like this instead:</p>
  
  <pre><code>John Smith &lt;john@foo.com&gt;
  </code></pre>
  
  <p>I've set up SPF and DKIM according to the MailGun instructions and everything passes as being correct.  I've also used <code>SetDKIM(true)</code> when I'm sending out the mail.  Is there some setting I'm missing or additional validation I need to do?</p>