Zimbra - Zimbra is trusted by over 500 million users to increase productivity with a complete set of collaboration tools while maintaining total control over security and privacy.

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

Microsoft Exchange Server - Microsoft Exchange Server is a messaging and collaborative software product developed by Microsoft.

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

Kolab Community - Kolab Groupware is a secure, scalable, reliable, flexible, full-featured 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.