Desktop email clients, and to a certain extent mobile native ones, can do this to a good enough approximation by changing one common setting. Dropbox magic is

Desktop email clients, and to a certain extent mobile native ones, can do this to a good enough approximation by changing one common setting. Dropbox magic is all in the scripts and combination of the scripts with hosting, rsync etc. Changing a setting is too much for many potential customers, but writing and/or maintaining an entire service all by yourself is qualitatively different.