After the latest update to 16.12.0 (KMail reports version 5.4.0) the notification icon is no longer visible in the notification area. There is an empty gap and if I hover over it with the mouse, the letter symbol fades in, and after leaving the mouse the symbol fades off. If the symbol is visible, there is also no longer a counter with the number of unread mails. That is rather annoying since you get no visible information of new and unread mails, if you were absent from your desk for a moment.
See also bug 373852. *** This bug has been marked as a duplicate of bug 371826 ***