This can often be tricky as you've probably done a variety of things (like tick the HTML box of the SMTP module), and it's still not working...

Steve Burge from OSTraining has written a great walkthrough of how to use Mimemail and Mailsystem to achieve the desired results...

http://www.ostraining.com/blog/drupal/html-emails-from-drupal-webform/

Good work Steve!