> http://help.cs.umn.edu/email/procmail http://www.courier-mta.org/maildrop/ http://www.courier-mta.org/maildrop/documentation.html http://www.courier-mta.org/maildrop/maildroptips.html http://www.wonkity.com/~wblock/docs/html/maildrop.html#_filtering_duplicate_messages