Bananatag - Find out what happens to your emails after you press send! Add email tracking, attachment tracking, email scheduling and templates to your daily emails.

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

GetNotify - GetNotify is a free email tracking software solution.

mlist - Email newsletters, outside of the inbox

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

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