Microsoft Outlook 2013, 2016 and Office 365 defaults to open email replies and forwards in the reading pane, to have the email pop-out by default instead in an uncluttered window, just follow the simple steps below.
Setup Outlook 2013 Exchange Manual Configuration – Office 365
The following guidelines step through the method of manually configuring your Office 365 Exchange Online account with Outlook 2010, 2013, and 2016 for Windows. This may be required when configuring secondary account or if you run into issues with the automatic configuration process.