Sanebox - SaneBox is a cloud-based service that allows users to filter and manage their emails.
Boomerang Respondable - Write perfect emails with Artificial Intelligence
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
Mailbutler - Mailbutler adds lacking functionality to email clients and increases your productivity
Ebsta - Capture every email and business contact inside Salesforce.
Wordzen - We write your emails for you. Literally.