Outlook - Envelope icon doesn't disappear

Hi,
I've enabled the Envelope Icon in the system tray for new mails (http://support.microsoft.com/kb/292034/en-us).

If a new mail arrives and I open Outlook 2013, read the mail, the envelope doesn't disappear, even if there aren't any unread messages.
The only temporary solution is to right-click any Outlook folder with all read mail and then selectMark All as Read in the context menu (mentioned here: http://superuser.com/questions/335122/why-does-the-outlook-taskbar-envelope-icon-sometimes-remain-active-even-if-i-ha)

We are using Outlook 2013 with Exchange 2010 SP3.

Why isn't the envelope hiding automatically, after I read all mails? This doesn't happen always, but most the times.

Thanks