▁with ▁version ▁ 4 . 0 . ▁ ▁The ▁client ▁software ▁was ▁also ▁named ▁Microsoft ▁Mail , ▁and ▁was ▁included ▁in ▁some ▁older ▁versions ▁of ▁Microsoft ▁Office ▁such ▁as ▁version ▁ 4 . x . ▁The ▁original ▁" In box " ▁( Ex change ▁client ▁or ▁Windows ▁Mess aging ) ▁of ▁Windows ▁ 9 5 ▁also ▁had ▁the ▁cap ability ▁to ▁connect ▁to ▁an ▁MS ▁Mail ▁server . ▁ ▁Microsoft ▁Mail ▁Server ▁was ▁eventually ▁replaced ▁by ▁Microsoft ▁Exchange ; ▁Microsoft ▁Mail ▁Client , ▁Microsoft ▁Exchange ▁Client , ▁and ▁Sch edule + ▁were ▁eventually ▁replaced ▁by ▁Out look ▁( Windows ▁and ▁Mac ). ▁ ▁Server ▁Architecture ▁Microsoft ▁Mail ▁was ▁a ▁shared - file ▁mail ▁system ; ▁the ▁" post office " ▁was ▁a ▁pass ive ▁database ▁of ▁files ▁which ▁could ▁res ide ▁on ▁any ▁file ▁server . ▁C li ents ▁used ▁mapped ▁network ▁drives ▁and ▁file ▁sharing ▁to ▁write ▁mail ▁to ▁the ▁post office . ▁C li ents ▁also ▁acted ▁as ▁Message ▁Transfer ▁Ag ents ▁( MT As ) ▁for ▁their ▁own ▁post off ices , ▁moving ▁around ▁messages ▁on ▁the ▁post office ▁as ▁needed , ▁including ▁queue ing ▁messages ▁for ▁out bound ▁delivery ▁to ▁other ▁post off ices , ▁and ▁processing ▁messages ▁que ued ▁as ▁arriv ing ▁from ▁external ▁sources . ▁ ▁Mail ▁that ▁needed ▁to ▁travel ▁between ▁post off ices ▁were ▁moved ▁by ▁an ▁external ▁M TA ▁called ▁External ▁( external . exe ), ▁which ▁originally ▁ran ▁on ▁MS - D OS . ▁A ▁version ▁of ▁External ▁for ▁OS / 2 ▁