Evolution/Empathy applet indicator disappears from Notification Area

The Evolution/Empathy applet indicator disappear from my Notification Area.
The notification aread is visible, and other Indicators Applets are still shown, the only Indicator missing is the little envelope that turns green when there is some some pending activity.

I am searching the web and Ubuntu site for a way to customize the Indicator Applet to solve this problem but found no information.
I used it to inform me of pending IM messages or received mails, and I really miss it.

How can I turn it on again?