For organisations looking for an alternative to Office 365 but a cloud solution for Enterprise Email, Opusflow recommend Zimbra.
Zimbra Collaboration Suite is an enterprise-class open source email, calendar and collaboration server. With the most innovative web application available today, Zimbra boosts the productivity of users on any desktop and dramatically reduces TCO (Total Cost of Ownership) compared to legacy platform vendors.
Other key advantages include advanced compatibility with existing desktop email clients, over-the-air sync to smartphones (iPhone, Android, BlackBerry) as well as better server scalability and more efficient administration.