I have a few distribution lists running in "Subscriber Only" mode, meaning that only somebody who is subscribed to the list (or on the Whitelist) can post to the list.
Some spammers have discovered the lists and are spoofing reply-to addresses of people on the list.
I've configured the Anti-spam settings to tag the messages matching "Weight20" in the headers with a {This is SPAM} message. It tags messages to users who have an account on the server just fine, but it's not flagging the mail going to the distribution lists.
Is SmarterMail bypassing List traffic? If so, is there a way to weed out these List-bound messages?
I don't see anything specifically that says you can filter mail lists, other than creating a white list of users who can post to the list. See this manual.
I would go towards James McFarland's suggestion of contacting SmartMail directly to see if they can help you.