He is doubting the hardware of the raspberry PI. Which you can over come by sheer numbers. For the same price as one "server" PSU you can buy 4 Rasberrys.
A full installation of Fedora has been ported onto it as a standard build.
Not that you would need that for a mail server.
Half a G of ram is more than enough.
They are mass produced and aren't soak tested before dispatch which server hardware would be. But for 35 quid and producing 30 000 of them a month what do you expect.
And for all the server hardware gets tested you still get the occasional wonky one.