hello,

if the option "Always show kmail in system tray" is checked, the kmail icon in system tray stays there even after the kmail window is closed. 

On the other hand, if the option "Only show KMail in system tray if there are unread messages" is checked and if there are no unread email, the KMail icon in system tray disappears after closing the main window, so that new incoming emails aren't notified until the app is manually relaunched. 

I believe KMail should not close completely if the "Only show KMail in system tray if there are unread messages" option is checked but should remain in the systray (hidden) so that new email may be notified.

Reproducible: Always

Steps to Reproduce:
1. Check the "Only show KMail in system tray if there are unread messages" option in KMail -> Settings -> Appearance System Tray
2. Make sure there are no unread emails
3. Close Kmail
Actual Results:  
The KMail icon disappears and the app is now completely closed, no new email will be notified until the main app is reopened

Expected Results:  
The KMail icon should remain hidden in the systray and the app should not completely close, as it happens when the "Always show kmail in system tray" is checked.