I'd love (and would pay for) a hosted email service that charged in some fashion other than per user or per domain. I've got various low volume addresses on various domains that I'd love to consolidate under one roof, and MS's custom domains was the closest thing to that. What keeps me from migrating to so many of these other services is the flat $x/address structure. That gets pricey fast if you have two or three per domain, and three, four, or more domains.
Other tacts might be charging by space consumed, messages sent, or a flat fee for a bucket with resource limits. Anyone know any providers offering models like that?