Where to find more discussion of e-mail filters:
Anonymous FTP:
ftp://ftp.halcyon.com/pub/ii/internet/filtering_mail_faq.txt and ftp://rtfm.mit.edu/pub/usenet/news.answers/mail/filtering-faq

Email:
Send mail to mail-server@rtfm.mit.edu containing the following:

send usenet/news.answers/mail/filtering-faq

UUCP:
uunet!/archive/usenet/news.answers/mail/filtering-faq

Hard Copy:
A printed version of that FAQ is in Chapter 25 of the book Internet Secrets by John R. Levine and Carol Baroudi; published 1995 by IDG Books; ISBN 1-56884-452-2.