, ▁but ▁the ▁proof ▁can ▁be ▁verified ▁efficiently . ▁For ▁email ▁uses , ▁a ▁text ual ▁encoding ▁of ▁a ▁hash c ash ▁stamp ▁is ▁added ▁to ▁the ▁header ▁of ▁an ▁email ▁to ▁prove ▁the ▁sender ▁has ▁exp ended ▁a ▁mod est ▁amount ▁of ▁CPU ▁time ▁calculating ▁the ▁stamp ▁prior ▁to ▁sending ▁the ▁email . ▁In ▁other ▁words , ▁as ▁the ▁sender ▁has ▁taken ▁a ▁certain ▁amount ▁of ▁time ▁to ▁generate ▁the ▁stamp ▁and ▁send ▁the ▁email , ▁it ▁is ▁unlikely ▁that ▁they ▁are ▁a ▁sp ammer . ▁The ▁receiver ▁can , ▁at ▁neg lig ible ▁computational ▁cost , ▁verify ▁that ▁the ▁stamp ▁is ▁valid . ▁However , ▁the ▁only ▁known ▁way ▁to ▁find ▁a ▁header ▁with ▁the ▁necessary ▁properties ▁is ▁br ute ▁force , ▁trying ▁random ▁values ▁until ▁the ▁answer ▁is ▁found ; ▁though ▁testing ▁an ▁individual ▁string ▁is ▁easy , ▁satisf actory ▁answers ▁are ▁rare ▁enough ▁that ▁it ▁will ▁require ▁a ▁substantial ▁number ▁of ▁tries ▁to ▁find ▁the ▁answer . ▁ ▁The ▁hypothesis ▁is ▁that ▁sp amm ers , ▁whose ▁business ▁model ▁re lies ▁on ▁their ▁ability ▁to ▁send ▁large ▁numbers ▁of ▁emails ▁with ▁very ▁little ▁cost ▁per ▁message , ▁will ▁ce ase ▁to ▁be ▁prof itable ▁if ▁there ▁is ▁even ▁a ▁small ▁cost ▁for ▁each ▁sp am ▁they ▁send . ▁Rece ivers ▁can ▁verify ▁whether ▁a ▁sender ▁made ▁such ▁an ▁invest ment ▁and ▁use ▁the ▁results ▁to ▁help ▁filter ▁email . ▁ ▁Techn ical ▁details ▁ ▁The ▁header ▁line ▁looks ▁something ▁like ▁this : ▁ ▁X - Hash c ash :