I use kontact --iconify as a startup program to immediately see if I have mail after boot via a notification. I would expect and like the iconify option to place kontact in the notification tray. Instead the window is opened minimized. I would prefer kontact to always minimize to the tray if that were possible.
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can confirm this behaviour still persist with KDE 5.25.5 and Kontact 5.21.1 there is also a old report at https://bugs.kde.org/show_bug.cgi?id=311666 hope it will be fixed.
Iconifying Kontact doesn't work at all in 5.22.2. All attempts to start it iconified (even via Konsole) fail, it doesn't even start minimized, let alone at the tray.