WHITE List vs BLACK List E-mail Filtering

COMPLEMENTARY WHITE AND BLACK LISTS

A 2-character field that can contain any combination of the characters 'A' and 'B' and 'C' has 9 potential combinations: AA AB AC BA BB BC CA CB CC.

Filter with a WHITE list containing entries AA AB AC and a BLACK list containing entries BA BB BC CA CB CC. In both cases, AA AB AC are classified 'WANTED' and BA BB BC CA CB CC are classified 'UNWANTED'.


PROOF

Use only the 10 characters '0' thru '9'. Possible configurations of a 2 character field are 10² = 100 ('00' to '99'). Possible configurations of a 3 character field are 10³ = 1000 ('000' to '999').

Mathematical induction teaches that if a formula is valid for 'n' and for 'n+1', then the formula is valid for all succeeding 'n's.