Spark Mail - Spark helps you take your inbox under control. Instantly see what’s important and quickly clean up the rest. Spark for Teams allows you to create, discuss, and share email with your colleagues

Mailtrack.io - The best email tracking solution I've found. Free

mlist - Email newsletters, outside of the inbox

Campaign Monitor - Email marketing software built for designers and their clients to run successful email campaigns.

TL;DR Email - Send short emails & read emails as fast as you read Facebook

Inbox Grader by Freshworks - Free metrics 📈 for your business and team email ✉️