Polymail - Native email app for email productivity.
Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.
HEY - Email at its best, new from Basecamp.
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
Canary Mail - Privacy-aware email app for Mac & iOS
MailClark - The Slack bot for external communications