Nylas Mail - The Nylas Cloud API powers your application with email, calendar & contacts features. Built-in features for better email, calendar, and contact management.
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
Morning Mail - A new way to manage your mail. Tinder for email.
MailClark - The Slack bot for external communications
Mailbird - Mailbird is the best email client for Windows 7, 8 and 10