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

Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.

mlist - Email newsletters, outside of the inbox

MailClark - The Slack bot for external communications

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

Hiver - Manage Customer Support, Sales and Projects right from your Gmail inbox. Collaborate with Shared Labels, Shared Contacts, email notes, email reminders and email templates