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.
Zendo - Zendo allows you to sell productized, subscription and custom services. Collaborate and collect payments. Create quotes and invoices. Eliminate multiple tools and enjoy the simplicity.
Roundcube - Web-based IMAP email client
Nextcloud Hub - On-premises, enterprise collaboration solution for documents, emails, calendars and more.
Microsoft Exchange Server - Microsoft Exchange Server is a messaging and collaborative software product developed by Microsoft.
GreenOrbit - Intranet software with everything you need, built in