Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.

Nylas Mail - The Nylas Cloud API powers your application with email, calendar & contacts features. Built-in features for better email, calendar, and contact management.

Roundcube - Web-based IMAP email client

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

ProtonMail - Secure email with absolutely no compromises. Get your free encrypted email account today.

Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.