Nylas Mail - The Nylas Cloud API powers your application with email, calendar & contacts features. Built-in features for better email, calendar, and contact management.
Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
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
Roundcube - Web-based IMAP email client
MailClark - The Slack bot for external communications
ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.