Email design really does fall under graphic design although the connection might not always be so obvious.
Business communication is largely done through email, and everyone knows how to operate their email, but not everyone is using their emails to the full potential.
While the primary objective of your email will be met, conveying information to the recipient, you could also gain additional benefit by having an attractive design and layout as well as provide information about your products and services.
What exactly you could be doing to increase the effectiveness of your emails does depend on your business, although a good start is to have a good looking design and include an informative and easy to read footer.
What you can use email design services for: