Steve Freegard’s new rule SMF_BRACKETS_TO seems pretty effective at catching certain recent spam campaigns, roughly 3% of common spam.  While the majority of this spam is already stopped by DNSBL’s, this may add a tiny bit of extra confidence in case an unlisted spammer gets through the network rules unscathed.

header SMF_BRACKETS_TO To:raw =~ /<<[^<>]+>>/
describe SMF_BRACKETS_TO Double-brackets around To header address