Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.
Sanebox - SaneBox is a cloud-based service that allows users to filter and manage their emails.
MailClark - The Slack bot for external communications
Thunderbird - Thunderbird is a free email application that's easy to set up and customize - and it's loaded with great features!
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
Ebsta - Capture every email and business contact inside Salesforce.