OBM - OBM is a free messaging and collaboration platform useful for just a couple to many thousands of users with strong support for mobile devices.
iRedMail - A fully fledged, free email server solution, an open source project (GPL v2).
Microsoft Exchange Server - Microsoft Exchange Server is a messaging and collaborative software product developed by Microsoft.
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.
mailcow - An open source mailserver suite.
DeskNow - Network Solutions - Original domain name registration and reservation services with variety of internet-related business offerings. Quick, dependable and reliable.