HTML / Mail System / SMTP Not sending approval in html
I believe drupal will only send text emails by default. You will need a module to help you implement HTML emails. I have used Mime Mail in the past to do this.

https://www.drupal.org/project/mimemail

From drupal.org:


  This is a Mime Mail component module (for use by other modules).
  
  
  It permits users to receive HTML email and can be used by other
  modules. The mail functionality accepts an HTML message body,
  mime-endcodes it and sends it. 
  If the HTML has embedded graphics,
  these graphics are MIME-encoded and included as a message attachment.
  Adopts your site's style by automatically including your theme's
  stylesheet files in a themeable HTML message format 
  If the
  recipient's preference is available and they prefer plaintext, the
  HTML will be converted to plain text and sent as-is. Otherwise, the
  email will be sent in themeable HTML with a plaintext alternative.
  Allows you to theme messages with a specific mailkey. Converts CSS
  styles into inline style attributes. 
  Provides simple system actions
  and Rules actions to send HTML email with embedded images and
  attachments.