UPDATE: I have mymacmail.com set up and I’m just waiting for the DNS entry to propogate. Right now I still have to create accounts manually. I plan to limit it to a few hundred users or less, not hundreds of thousands of users. If it gets too big I’ll probably end up outsourcing it to Outblaze or one of the other email services.
Right now it doesn’t cost me anything to run (other than the $15/year to register the domain), so I can offer it as a free service.