imate ▁e - mail ▁that ▁no ▁real ▁money ▁is ▁involved . ▁Ne ither ▁the ▁sender ▁nor ▁recip ient ▁need ▁to ▁pay , ▁thus ▁the ▁administrative ▁issues ▁involved ▁with ▁any ▁mic rop ay ment ▁system ▁and ▁moral ▁issues ▁related ▁to ▁charg ing ▁for ▁e - mail ▁are ▁entirely ▁avoided . ▁ ▁On ▁the ▁other ▁hand , ▁as ▁Hash c ash ▁requires ▁potentially ▁significant ▁computational ▁resources ▁to ▁be ▁exp ended ▁on ▁each ▁e - mail ▁being ▁sent , ▁it ▁is ▁somewhat ▁difficult ▁to ▁t une ▁the ▁ideal ▁amount ▁of ▁average ▁time ▁one ▁wishes ▁clients ▁to ▁exp end ▁computing ▁a ▁valid ▁header . ▁This ▁can ▁mean ▁sacrific ing ▁access ibility ▁from ▁low - end ▁embedded ▁systems ▁or ▁else ▁running ▁the ▁risk ▁of ▁host ile ▁hosts ▁not ▁being ▁challeng ed ▁enough ▁to ▁provide ▁an ▁effective ▁filter ▁from ▁sp am . ▁ ▁Hash c ash ▁is ▁also ▁fairly ▁simple ▁to ▁implement ▁in ▁mail ▁user ▁agents ▁and ▁sp am ▁filters . ▁No ▁central ▁server ▁is ▁needed . ▁Hash c ash ▁can ▁be ▁increment ally ▁deployed — the ▁extra ▁Hash c ash ▁header ▁is ▁ignored ▁when ▁it ▁is ▁received ▁by ▁mail ▁clients ▁that ▁do ▁not ▁understand ▁it . ▁ ▁One ▁pla us ible ▁analysis ▁concluded ▁that ▁only ▁one ▁of ▁the ▁following ▁cases ▁is ▁likely : ▁either ▁non - sp am ▁e - mail ▁will ▁get ▁stuck ▁due ▁to ▁lack ▁of ▁processing ▁power ▁of ▁the ▁sender , ▁or ▁sp am ▁e - mail ▁is ▁bound ▁to ▁still ▁get ▁through . ▁Ex amples ▁of ▁each ▁include , ▁respectively , ▁a ▁central ized