Improve Email Marketing Using Principles of UX Design http://isenselabs.com/posts/how-to-improve-email-marketing-using-principles-of-ux-design
The psychology of colors in email marketing https://www.mailjet.com/blog/psychology-of-colors-in-email-marketing/
The Importance Of Responsive Design in Emails https://blog.getresponse.com/the-importance-of-responsive-design-in-emails.html
How design adds value to a simple notification e-mail http://www.proxyclick.com/blogs/proxyclick-journey/a-story-of-how-design-adds-value-to-a-simple-e-mail