sp ent ▁Hash c ash ▁st amps . ▁However , ▁although ▁the ▁hash c ash ▁plugin ▁is ▁not ▁initially ▁on ▁by ▁default , ▁it ▁still ▁needs ▁to ▁be ▁configured ▁with ▁a ▁list ▁of ▁address ▁patterns ▁that ▁must ▁match ▁against ▁the ▁Hash c ash ▁resource ▁field , ▁so ▁it ▁doesn ' t ▁actually ▁work ▁by ▁default . ▁ ▁Email ▁clients ▁The ▁Pen ny ▁Post ▁software ▁project ▁on ▁Source For ge ▁implements ▁Hash c ash ▁in ▁the ▁Moz illa ▁Th under bird ▁email ▁client . ▁The ▁project ▁is ▁named ▁for ▁the ▁historical ▁avail ability ▁of ▁conventional ▁ma iling ▁services ▁that ▁cost ▁the ▁sender ▁just ▁one ▁pen ny ; ▁see ▁Pen ny ▁Post ▁for ▁information ▁about ▁such ▁ma iling ▁services ▁in ▁history . ▁ ▁Email ▁Post mark ▁Microsoft ▁also ▁designed ▁and ▁implemented ▁a ▁now ▁deprecated ▁open ▁spec , ▁similar ▁to ▁and ▁yet ▁in compatible ▁with ▁Hash c ash , ▁Email ▁Post mark , ▁as ▁part ▁of ▁their ▁Co ordin ated ▁Sp am ▁Red u ction ▁In iti ative ▁( CS RI ). ▁The ▁Microsoft ▁email ▁post mark ▁variant ▁of ▁Hash c ash ▁is ▁implemented ▁in ▁the ▁Microsoft ▁mail ▁infrastr ucture ▁components ▁Exchange , ▁Out look ▁and ▁Hot mail . ▁The ▁format ▁differences ▁between ▁Hash c ash ▁and ▁Microsoft ' s ▁email ▁post mark ▁is ▁that ▁post mark ▁hash es ▁the ▁body ▁in ▁addition ▁to ▁the ▁recip ient , ▁and ▁uses ▁a ▁modified ▁S HA - 1 ▁as ▁the ▁hash ▁function ▁and ▁uses ▁multiple ▁sub - p uzz les ▁to ▁reduce ▁proof ▁of ▁work ▁variance . ▁