I am paying for personal email, internet connectivity, virtual servers and other services, and I don't think I would ever use such a service for personal purposes that isn't flat-rate. I don't actually use a lot of resources; I just wouldn't want abuse, security issues or other irregularities to threaten my ability to eat.

I'm not sure it's a winning proposition to have variable rate email services aimed at individuals or small companies.

People prefer to know up front what something costs, and I imagine they even rather pay double or triple the "real cost" if it means they don't have to think about it any more.