Email Design Guide from MailChimp

http://mailchimp.com/resources/email-design-guide

Improve Email Marketing Using Principles of UX Design

http://isenselabs.com/posts/how-to-improve-email-marketing-using-principles-of-ux-design

UX in Email Design

http://www.webdesignerdepot.com/2016/01/ux-in-email-design/

Adding Interactivity to Email Design

http://www.emailmonks.com/interactive-email-advancement.html

The psychology of colors in email marketing

https://www.mailjet.com/blog/psychology-of-colors-in-email-marketing/

The Dawn of Kinetic Email

http://freshinbox.com/blog/the-dawn-of-kinetic-email/

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

Email is User Experience

http://customer.io/blog/email-whole-user-experience.html