getting amavis to notify spam senders ONLY if they are internal users?

I'm trying to get amavis to notify people if they're blocked by the spam filters, but only if they're valid internal users. I would have thought that putting
$final_spam_destiny = D_BOUNCE;

into the myusers group in amavis/conf.d/50-user would do the trick, but this sends a bounceback to external spammers as well.

Is such a thing even possible?