Polymail - Native email app for email productivity.
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
Airmail - Airmail is a lightweight and lightning fast mail client for Mac.
Superhuman - Superhuman is an email management tool.
Morning Mail - A new way to manage your mail. Tinder for email.
Mailman - Mailman is free software for managing electronic mail discussion and e-newsletter lists.