Why does this qdbus command work and my dbus-send command not?

[vatbier]

KMail Version 1.10.1
Using KDE 4.1.2

In the kde-pim at kde.org mailing list Thomas McGuire gave me a working qdbus command on my question why my dbus-send command for kmail didn't work:
http://lists.kde.org/?l=kde-pim&r=1&b=200811&w=2 : kmail dbus method to hide in systray doesn't work

This command doesn't work:
dbus-send --type=method_call --dest=org.kde.kmail
 /kmail/kmail_manwindow_1 com.trolltech.Qt.QWidget.setHidden boolean:true

This command does work:
qdbus org.kde.kmail /kmail/kmail_mainwindow_1 setHidden true

It hides the kmail window to systray (you must have enabled the kmail setting for the kmail system tray icon).

I can't see what I'm doing wrong with my dbus-send command.
I can't find better info on how to use dbus-send.
I can't find no info at all on how to use qdbus, is there any tutorial?

vatbier