> There really needs to be a zero cost custom-domain email hosting solution.Unfortunately I doubt that would work. Any email solution hoping to get significant use will need to be able to send as well as receive and/or store & organise, because if someone is needing to run their own send route elsewhere they'll run their own receive route there too, and any free email solution that allows sending will be abused in any way possible by junk mailers and other scammy types.
It would be a constant fight to keep the service off black-lists and dealing with the backlash from:
* people blocked rightly because of service abuse
* people blocked because of a false positive in what-ever automated checks you do (the admin involved in doing all the checks manually will be excessive enough that it can't all be done manually)
* those who are inconvenienced by the service, even temporarily, being on a blacklist
And that mass of spammers such a service will attract will be worthless: they'll have everything quickly automated and won't even glance at a single one of the adverts.
> With $20/user/year, there should be no difficulty paying the hosting costs
I doubt $20/user/year would cover it unless you get a decent number of subscribers. And I doubt there would be enough people who would sign up for this, who are not already using gmail or similar big providers that already exist or mail provision included with their web hosting or domain registration service.
Of course my gut reaction could be quite wrong here: why don't you give it a go? If you are right then there is a market to corner that no one else is looking at yet so you could make it big.