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.

Todoist - Todoist is a to-do list that helps you get organized, at work and in life.

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

Todo.txt - Track your tasks and projects in a plain text file, todo.txt. A todo.

Roundcube - Web-based IMAP email client

EssentialPIM - EssentialPIM is a free Personal Information Manager that keeps up with the times and lets you manage appointments, tasks, notes, contacts, password entries and email messages across multiple devices and cloud applications.