@freemo "Man" also means human in it's other meaning,
HUman or MANkind.
(there are more meanings)
It may have been the original meaning (I'm assuming / guessing / relying on history a bit here that all mail personnel were mostly male (say >80% men, even more probably like 99% at some stage) and probably still is a high number... so it's not too bad as a word lasting the changes in human time and human history.
I can guess the post was a joke and a reality etc in one so I'm just focussing on the reality part as many do hear 'man' more than HUman or Man as in MANkind'.