I note during this recent onslaught of PDF-based image spam that virtually all of the PDF attachments are less than 20 or 30k, which is considerably smaller than any legitimate PDFs that I might receive. It seems that if I can set a rule in Mail that, among other things, detects a PDF that's less than a given size, Mail could then recognize the message as spam. The problem is, i don't see anything in the rules configuration panel that lets me use attachment size as a criterion. Is this possible in Mail?