For many years Microsoft Outlook was the most popular email client in the world. According the latest research, Microsoft Exchange Server accounts for more than half of business mailboxes in 2014; that is, 500m in absolute figures. Many of these users are using Microsoft Outlook, where Outlook Web Access still plays a companion role.
The installation base of Microsoft Office is about 1 billion. This figure can be found in many articles. On July 7, 2014 Microsoft reported that over 1.1 billion people use Microsoft Office. So the estimated base of Microsoft Outlook users in 2014 is between 500m and 600m.