Eh, I don't buy it. Sure, searching on an storing a lot of mail is a tough problem, but there are plenty of startups working on what I'd consider to be much harder. With open source tools, AWS, etc. this isn't an impossible problem. Plus, you don't have to scale to millions of users right away.
I think people are staying away from this area for a reason. Email is of declining importance anyway. I know more and more people that, for better or worse, communicate mainly through Facebook.
Also, don't underestimate how much work it is to filer spam effectively.
more and more people using facebook sounds like a reason and an opportunity to fix email. why is it easier to send messages via facebook?
email has a few huge disadvantages: you need to keep your address book up to date and your inbox is full of email from machines - notifications and bulk.
make an email provider that has a twist - maybe automatically sync it with your facebook friends as the address book and limit lenght of your replies.
