The NY Times reports that spam has simply exploded over the last year (especially the last few months):

You’re not the only one. Spam is back -- in e-mail in-boxes and on everyone’s minds. In the last six months, the problem has gotten measurably worse. Worldwide spam volumes have doubled from last year, according to Ironport, a spam filtering firm, and unsolicited junk mail now accounts for more than 9 of every 10 e-mail messages sent over the Internet.

That kind of volume is going to kill free email. I'm not sure how a paid system would push itself onto the net, but I would bet that a paid (and open only to members) service by a large vendor would attract interest.