>dkim can help as one component of a content filtering solution. Current versions of Spamassassin can do DKIM checking. Don't turn on ADSP "reject because I say so" checks (I say this as one of the authors of the ADSP RFC), but you can adjust your config to list a few heavily phished DKIM signers like paypal.com and ebay.com on which you can reject unsigned mail with little collateral damage.
If you're already using spamassassin, you just add stuff to local.cf as suggested by comments in the DKIM plugin, and then restart spamd. R's, John
