no matter if I change the print font setting kmail uses the same font for printing.
Verified. I'm almost sure upstream won't fix such a minor issue in KDE 3.5 anymore and personally my sympathy is with the trees, so unless someone else is interested to provide a patch, you'll have to live with it.
This is an upstream issue so we can't do anything and you can't report it, because 3.5 development is frozen. But you could try 3.5.10.