"I often see people saying 'Nobody has produced an invulnerable software system; therefore, nobody will ever produce an invulnerable software system.'"By the same bogus reasoning, nobody will ever reach Mars; nobody will ever find MD5 collisions; nobody will ever cure cancer; nobody will ever prove the Poincare conjecture; nobody will ever clone a human; nobody will ever build a 1GHz CPU; nobody will ever find SHA-1 collisions; nobody will ever break the sound barrier; etc."
"So it's tempting to incorporate a smaller resolver library into qmail. […] I'd no longer be able to blame the BIND authors and vendors for the fact that attackers can easily use DNS to steal mail." [From the file "THOUGHTS" of the qmail distribution]
The great thing about attackers is that there are so many to choose from!