Excellent guide to responsive email design. See what is possible with responsive email design, how to implement media queries in your design, and support for responsive emails in various mobile email applications.
Email v. Social Media Infographic: An epic showdown between two of the most widely used digital marketing platforms. Which one deserves your marketing dollars? Spoiler Alert: Email Marketing Knocks Out Social Media in 5 Rounds
This infographic by the Monks Monastery depicts an understanding of A/B Split Testing & Multivariate Testing along with testing best practices, elements to test, etc. for effective email campaign testing.