I sent an email to plugins [at] artyshow-studio DOT fr. Not sure if you received it.
In that email, I attached a patch to add a plain-text template to the admin screen so site admins can customize the plain-text email as well.
Let me know if you received it or if you have a different email address you want me to send the patch to.
I would really love it if you could include the code into the next version of your awesome plugin!