Email Client Market Share - Most popular clients to ensure you design for based on Litmus Email Analytics. Clients are aggregated, so if there is a concern with a paticular version of a client, further analysis required. For example, if you see an issue on version 5.x of Android (in the month of October 2016), you can see the version represents about 30% of Androids, therefore it impact about 3% (30% of 10% market share of Androids).
Email Client CSS Support / Email Design Reference - CSS reference chart for various email clients.
CSS Support Guide for Email - A complete breakdown of the CSS support for the top 10 most popular mobile, web and desktop email clients on the planet.
The Fab Four Techinque - Responsive Emails without Media Queries