Rule for non-DKIM-signed messages

[Kurt Fitzner]

I've noticed on my mail server that DKIM signing is almost diagnostic of 
spam.  Almost no legitimate sender is without DKIM, and about 90% of my 
spam is unsigned, so I want to bias non-DKIM-signed heavily towards 
spam.  To that end I was wondering if there are any built-in rules I can 
activate to score emails that are not DKIM-signed? I'd rather use a 
built-in rule than roll my own.