Targeted.io - Test your email designs across 30+ clients before you send'm
Sketch Tricks - Articles for Sketch
Really Good Emails - A large collection of good product email design.
The Good Workshop - TGW helps experts plan, run, and track UX, CX, Product and innovation workshops, fully automated and straightforward user experience.
Stamplia - High converting and beautifully designed email templates
Git Sketch Plugin - Version control for designers