We had a requirement to send an HTML formatted email from our application. Unfortunately, using the Email action along with the an EmailBody field containing HTML did not work -- the HTML is not parsed and shows up within the email client.
I stumbled upon a way to get this working.
You'll need a control with an action of Click to Native Browse
Set the Target Control to a Text Box (can be bound to a source object or not)
Set the value (can be set via LUA script, default value or formulas, whatever suits your specific needs) with the following format:
mailto:email@example.com?subject=Your Subject&body=<html><body><b>Bold text!</b></body></html>
Hope this helps!