When you access your mailbox by using Outlook Web App in a Microsoft Exchange Server 2013 environment, the first email message is selected and marked as read automatically.
This issue occurs because of a by design feature in an Exchange Server 2013 environment.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
Article ID: 3056817 - Last Review: Jun 16, 2015 - Revision: 1
Exchange Server 2013 Enterprise, Exchange Server 2013 Standard Edition