Just in case one would like to attach additional spam score to faked twitter messages with original twitter text, faked signatures, and actual URL links to spam sites, here are the rules (for SA 3.3.*, DKIM plugin enabled):
full __L_DKIM_VALID_TWITTER eval:check_dkim_valid(twitter.com) header __L_FROM_TWITTER From:addr =~ /[\@.]twitter\.com$/mi meta L_FAKE_TWITTER __L_FROM_TWITTER && !__L_DKIM_VALID_TWITTER score L_FAKE_TWITTER 5 Mark
