As one of the companies bought out by AB we used to do domestic german mail flights every night on our 733s, however the majority of the night air mail system was flown by lufthansa. Nowadays all domestic mail flights in germany have been discontinued and its all done on the ground (trucks, railway-freight). However international mail is still flown apparently, although i thought the DHL fleet (and its subcontractors) do most of it.
We had special seat containers to load the mail into (basicly canvas covers for the pax seats) additionaly the mail could be loaded into the overhead bins and of course they filled up first all the cargo holds. In a 733 we could transport somewhere between 12 and 15 tons of mail depending on how well those mail boxes were filled. However we did not do mixed pax/mail flights, only pax we did transport was crews on a deadhead leg as the first row was empty and available to them.
I have to say i kinda miss flying mail, quite different on the cabin side than passengers and pretty relaxing compared to them.