Microsoft and Yahoo! are currently considering methodologies for applying fees for email use and strongly believe it will deplete, if not eradicate, spam and junk email.
Email users would be expected to purchase e-stamps which would identify where emails were originating, sourcing any large spam operatives. E-stamps may cost as little as 1 cent, still making email affordable for the average internet user. Read more of the issues involved here.
Following from Bill Gates announcement, Yahoo! says it is experimenting with an "economic friction" model to deter spamming, but says it will continue to provide a free web email service. Read more about their economic friction strategy. Further reading can be obtained here