LazyWeb: Incoming mail with > n "To:" recipients?

Related to “Thanks. No.” and email filtering, I wonder how hard it would be for Mail.app, etc. to have a rule by which messages with more than n recipients in the “To:” line could be flagged for (depending on your preferences and courage) filtering, auto-archiving, or deletion. Maybe via AppleScript?

I’ve heard from several friends who filter all non-work email for which they aren’t the exclusive “To:” recipient, but it would be handy to have some flexibility in what your own magic number is – plus of course what you’d then do with emails that exceed your limit would be up to you.

But in an edge case, for example, if I get an email that went to [>=90 TO: recipients] and [<=25%] of the recipients were in my Address Book, the message would be flagged as “possible friend spam.” (And, yes, I was once on a “Hey, this is funny” list that went to 96 people multiple times each day. Good times.)

So, any thoughts? Bonus points if it’s a rule that’s easy for non-geeks to recreate in GUI apps like Mail.app, Entourage, and Outlook, etc. Comments open for brainstorming.

(In related news, as I mentioned on MM.com, I’ll soon be opening a thread on the Board to take suggestions on improving Thanks. No., so keep your powder dry on that one.)