The idea that households would pay $50/month for email is a bit of a stretch given that the alternative is free email that works pretty damn well.
I am talking about this service that pg thinks needs to replace email. I do agree with him that people currently use email in a way it was not intended to (todo list, sending files, etc), and that surely there must be a better way of doing these tasks, that would be worth $50/month, while getting rid of the limitations of email such as attachment size limits.
In fact I know I am right, because the reason we, in 2012, still have no easy way to share large files between friends and family members, is precisely because the storage and bandwidth costs are higher than the cost of email.
How many services do you yourself pay for today on a monthly basis? I can count mine on one hand. To get $50 from me every month you better have a life-changing service.
That's only $10/month/user for a family of five. Come up with reasonable limits to ensure enough revenue (eg. max 10 users per plan).