David Ferris Reason Quotations

Another reason is that many users will have restrictions on their mailbox sizes for the foreseeable future, ... A flood of spam can cause the message store space to run out, in which case new bona fide incoming e-mail will be refused.

The trouble is, almost all filters today work using simple keyword-matching approaches, when what's needed is something with more natural language interpretation intelligence. Otherwise, any reasonable attempt at defining filters ends up either letting much too much in that you don't want in, or stopping bona fide messages.