ced ▁by ▁the ▁processing ▁time ▁required ▁to ▁generate ▁the ▁Hash c ash ▁string . ▁However , ▁sp amm ers ▁would ▁suffer ▁significantly ▁due ▁to ▁the ▁large ▁number ▁of ▁sp am ▁messages ▁sent ▁by ▁them . ▁ ▁Re cip ient ' s ▁side ▁Techn ically ▁the ▁system ▁is ▁implemented ▁with ▁the ▁following ▁steps : ▁ ▁The ▁recip ient ' s ▁computer ▁calcul ates ▁the ▁ 1 6 0 - bit ▁S HA - 1 ▁hash ▁of ▁the ▁entire ▁string ▁( e . g ., ▁). ▁This ▁takes ▁about ▁two ▁micro seconds ▁on ▁a ▁ 1   G Hz ▁machine , ▁far ▁less ▁time ▁than ▁the ▁time ▁it ▁takes ▁for ▁the ▁rest ▁of ▁the ▁e - mail ▁to ▁be ▁received . ▁If ▁the ▁first ▁ 2 0 ▁bits ▁are ▁not ▁all ▁zero , ▁the ▁hash ▁is ▁invalid . ▁( L ater ▁versions ▁may ▁require ▁more ▁bits ▁to ▁be ▁zero ▁as ▁machine ▁processing ▁spe eds ▁increase .) ▁ ▁The ▁recip ient ' s ▁computer ▁checks ▁the ▁date ▁in ▁the ▁header ▁( e . g ., ▁, ▁which ▁represents ▁the ▁date ▁ 8 ▁A pr ▁ 2 0 0 6 ). ▁If ▁it ▁is ▁not ▁within ▁two ▁days ▁of ▁the ▁current ▁date , ▁it ▁is ▁invalid . ▁( The ▁two - day ▁window ▁compens ates ▁for ▁clock ▁ske w ▁and ▁network ▁routing ▁time ▁between ▁different ▁systems .) ▁ ▁The ▁recip ient ' s ▁computer ▁checks ▁whether ▁the ▁e - mail ▁address ▁in ▁the ▁hash ▁string ▁matches ▁any ▁of ▁the ▁valid ▁e - mail ▁addresses ▁registered ▁by ▁the