[Exim] a spam bcc filter?

I was wondering if anyone had any quick comments on a filter that would block mail addressed to a non-local user with bcc
addresses. It seems that some of the spam we receive is to 'somename@???' with hundreds of users listed in the bcc.
Is there a reason I would not want a filter like this enabled? Does anyone have a better idea? I am *really* new at
filters with yesterdays iloveyou virus being the first day in filters 101. :)

tia,
-jm