title: symfony 2 swiftmailer Cannot send message without a sender address
body: <p>Trying to configure SwiftMailer to send messages with gmail.</p>
  
  <p>Here is my configuration:</p>
  
  <pre><code># Swiftmailer Configuration
  swiftmailer:
      transport: gmail
      host:      ~
      username:  username@gmail.com
      password:  password
  </code></pre>
  
  <p>And my controller action:</p>
  
  <pre><code>     public function registerAction(Request $request) {
                      $message = new \Swift_Message();                    
                              $message::newInstance()
                                -&gt;setFrom('username@gmail.com')
                              -&gt;setSubject('ssssssss')                            
                              -&gt;setTo(array('username@gmail.com'))
                              -&gt;setBody(
                                      'text is going here');       
                              $res = $this-&gt;get('mailer')-&gt;send($message);
  }
  </code></pre>
  
  <p>And when I execute it:</p>
  
  <p><strong>Cannot send message without a sender address
  500 Internal Server Error - Swift_TransportException</strong></p>
  
  <pre><code>Stack Trace
  in /var/www/local/symfony/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php at line 164   
          }
          if (!$reversePath = $this-&gt;_getReversePath($message)) {
              throw new Swift_TransportException(
                  'Cannot send message without a sender address'
                  );
          }
  </code></pre>