Horde - Horde Groupware is a free, enterprise ready, browser based collaboration suite.
Fantastical 2 - Fantastical, the Mac calendar app you'll enjoy using. Quickly create new events with natural language input and more.
Roundcube - Web-based IMAP email client
Microsoft Outlook - Organize your world. Outlook’s email and calendar tools help you communicate, stay on top of what matters, and get things done.
Mailo - Mailo is an email client where you can send and receive emails to and from anyone with an email address.
Morgen.so - All-in-one Calendar, Tasks & Scheduler. Morgen is the single hub for everything that revolves around time management.