You sign in to Outlook Web App by using Windows Internet Explorer 8.
You create an email message in plain text, and then you type some text in the message body.
You click another window. For example, you click a new tab in Internet Explorer 8, a notepad file, the desktop, and so on.
In this scenario, the message body is removed in Outlook Web App.
To resolve this issue, install the following update:
2936880 Cumulative Update 5 for Exchange Server 2013
To work around this issue, use the HTML format as the Outlook Web App default format.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information about the Set-MailboxMessageConfiguration cmdlet, go to the following Microsoft website: