Whenever I post a tutorial about sending emails using Drupal, the first question I get asked is how to send emails as HTML. By default, emails are sent as plain text in Drupal 7. There are a few modules out there that allow you to send emails as HTML like Mime Mail, HTML Mail and Swift Mailer.
There are a few benefits in sending emails as HTML. For one thing, they can be styled and images can be displayed. Also, it's easier to track open rates and clicks if the email is HTML. However, there are some downsides to using HTML. One thing I've read about is that HTML emails are more likely to be flagged as spam. Especially if your whole email is just a single image.