Changing default email client system wide in kde

[Roderick Johnstone]

Hi

I'd like to change the default email client in kde from kmail to 
thunderbird.

I can change it for my pown account by using system settings / Default 
applications, and it seems to add a file emaildefaults in my .kde tree, 
but I'd like to be able to change it system-wide for all my users.

Copying that emaildefaults to /usr/share/config doesnt seem to work.

Can anyone suggest the correct way to effect this change of email client 
please.

Thanks

Roderick