This would be very easy for ISPs to set up on their leased routers. In fact until recently most people would have their email come from their ISP ! Some would also have a personal website space - mine is somehow still up 20 years later ! (At the time, routers wouldn't be common, especially not full-blown Linux computers, so it was all on ISP's servers.)
The only tricky part (which is perhaps even worse already with the "cloud"), is that some legislation is needed to simplify transfers (and not just the file ones) from one ISP to another, like we already have with phone numbers.
The only tricky part (which is perhaps even worse already with the "cloud"), is that some legislation is needed to simplify transfers (and not just the file ones) from one ISP to another, like we already have with phone numbers.