This issue occurs when you compose an email message in Outlook Web App by using Internet Explorer 10, Internet Explorer 9, or Internet Explorer 8, and move the focus from the message subject to the body by pressing Tab on the keyboard.
To resolve this issue, install the following update:
2936880 Cumulative Update 5 for Exchange Server 2013
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
For more information about Input method editors (IMEs), go to the following Microsoft website: