I have thought a lot about this whole email-based service thing and its viability as a business. I'm not sure its something I'd want to go into.
I made something that is uses email for 95% of my interactions with it. It allows me to remove some friction between email and other activities by setting up forwarding addresses that can be configured to do one of three things:
1) Forward a receipt to a unique address that will convert the receipt to a pdf and forward the pdf as an attachment on to our bookkeeper.
2) Forward any email to a unique address and my tool will post the email + attachments to a Campfire room that is associated with the unique address.
3) Forward any email + attachments to a unique address and the email will be converted to PDF, then posted with the attachments to a folder in my Dropbox account that I associated with the address.
It is incredibly useful. It saves me and my people minutes of time every day and I've considered releasing it to the masses.
Here's the rub...I'm not interested in just giving it away since its a set it and "it just works" solution, so advertising is unlikely to cover hosting, let alone support or ongoing development. People may pay for it, but there are too many examples of mostly email-based services going belly up.
I believe these email-type services are just hard to monetize, even if they are useful and in demand. It is hard to keep something going when each new user means more money down the tube.