At $99 per 40GB HDD, there is no excuse for lack of space.

And the source for this price point for a SPARC server would be?

40GB also doesn't scale well to 5,000,000 users.

So let's say we double our user's boxes from 10MB to 20MB.

10MB X 5,000,000 = 50TB (assuming only one mail box per customer, more like
two plus in the real world so you might consider trebling the following
figures in the real world)

50TB/40GB = 1,250 HDD X $99 = $123,750 (at your PC price) plus the not
inconsiderable cost of the servers/raids to hold them and staff to
administer/maintain them. My WAG (wild ass guess) would place the true
multiple E-mail box/user cost at about $1/customer/month; close to, if not
more than, most ISP's profit margin. (Profit, what profit?)

Now if we can just get the marketing folk to do a study to see if our
customers would be willing to pay that additional $1/month to fund this in a
market where more and more providers are offer additional "free" mail