[Simucal]

I think the fee could be applied only to unsolicited emails. If I have a particular email address whitelisted then it will get through my gateway with no fee attached. However, if it is an unsolicited email from someone not on my whitelist it will be required to have some small fee attached to get past my filter.

I've seen some suggestions that instead of using money you could use some proof-of-work computation. This is something we could scale the difficulty factor of as computing power increased. Something that takes, say, an average computer 30 seconds (or longer) to calculate. It will attach this proof-of-work to the email.