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
Polymail - Native email app for email productivity.
TL;DR Email - Send short emails & read emails as fast as you read Facebook
Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.
mlist - Email newsletters, outside of the inbox
Email Inspector - Clean-up your email list