Salamat DwiwiN,
I'm not sure if this is a newbie or wish-list question, but after several hours of searching the forums and ploughing through qmail man pages I can't come up with a quick solution either.
If someone already using the dungog.net email management add-in can confirm this is possible using that add-in I'll go download it asap, but from the description:
"Standard email delivery options are local, forward or both" - it's not totally clear if that actually limits users to internal email only.
I know it's a bit harsh to limit some people's email but it has to been done often enough I'm surprised there's not a quick way. Unless I'm missing something obvious of course

thanks in advance, -steve.