HTML Emails don't display correctly (through SMTP)

I have the same problem in the 2 issues:
http://drupal.org/node/131523
http://drupal.org/node/134714

I use smtp 5.x-1.x-dev to send mails through GMail and I test 3 scenarios:
1. Register new user, system will send a mail to new user
2. Use this module to forward a node
3. User Subscriptions module to mail comments to author

But I found only forward module sends mail with "DOCTYPE ...". I checked the source code of forward and SMTP module, I guess the reason is SMTP thinks the mail from forward module is "text", not "html". So, I changed the source code of forward, it's ok now. But I'm not sure is it a correct solution.

Original:

  $headers['MIME-Version'] = '1.0';
  $headers['Content-Type'] = 'text/html; charset=utf-8';

Modified:

  $headers['MIME-Version'] = '1.0';
  $headers['Content-Type'] = 'text/html';