Nylas Mail - The Nylas Cloud API powers your application with email, calendar & contacts features. Built-in features for better email, calendar, and contact management.
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
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
MailClark - The Slack bot for external communications
InstAddr - Disposable email address application.