The best alternatives to SmashBack are mjml., Design Triggers, and Email Christmas Tree. If these 3 options don't work for you, we've listed a few more alternatives below.
MJML is a markup language designed to reduce the pain of coding a responsive email. Its semantic syntax makes it easy and straightforward and its rich standard components library speeds up your development time and lightens your email codebase.
Use timetoreply Email Analytics on top of your existing email client to get real-time reporting, alerts and track your team's email reply times to important emails