▁recip ient , ▁or ▁matches ▁any ▁of ▁the ▁ma iling ▁lists ▁to ▁which ▁the ▁recip ient ▁is ▁subscri bed . ▁If ▁a ▁match ▁is ▁not ▁found , ▁the ▁hash ▁string ▁is ▁invalid . ▁ ▁The ▁recip ient ' s ▁computer ▁inser ts ▁the ▁hash ▁string ▁into ▁a ▁database . ▁If ▁the ▁string ▁is ▁already ▁in ▁the ▁database ▁( ind ic ating ▁that ▁an ▁attempt ▁is ▁being ▁made ▁to ▁re - use ▁the ▁hash ▁string ), ▁it ▁is ▁invalid . ▁ ▁If ▁the ▁hash ▁string ▁passes ▁all ▁of ▁these ▁tests , ▁it ▁is ▁considered ▁a ▁valid ▁hash ▁string . ▁All ▁of ▁these ▁tests ▁take ▁far ▁less ▁time ▁and ▁disk ▁space ▁than ▁receiving ▁the ▁body ▁content ▁of ▁the ▁e - mail . ▁ ▁Re quired ▁effort ▁The ▁time ▁needed ▁to ▁compute ▁such ▁a ▁hash ▁collision ▁is ▁exponential ▁with ▁the ▁number ▁of ▁zero ▁bits . ▁So ▁zero ▁bits ▁can ▁be ▁added ▁( d ou bling ▁the ▁amount ▁of ▁time ▁needed ▁to ▁compute ▁a ▁hash ▁with ▁each ▁additional ▁zero ▁bit ) ▁until ▁it ▁is ▁too ▁expensive ▁for ▁sp amm ers ▁to ▁generate ▁valid ▁header ▁lines . ▁ ▁Conf irm ing ▁that ▁the ▁header ▁is ▁valid ▁is ▁very ▁much ▁faster ▁and ▁always ▁takes ▁the ▁same ▁amount ▁of ▁time , ▁no ▁matter ▁how ▁many ▁zero ▁bits ▁are ▁required ▁for ▁a ▁valid ▁header , ▁since ▁this ▁requires ▁only ▁a ▁single ▁has hing ▁operation . ▁ ▁Adv antages ▁and ▁dis adv antages ▁ ▁The ▁Hash c ash ▁system ▁has ▁the ▁advantage ▁over ▁mic rop ay ment ▁propos als ▁applying ▁to ▁legit