Bill Gates suggests to stop providing email services for free and instead make people buy (virtual) stamps for each and every email:
If the U.S. Postal Service delivered mail for free, our mailboxes would surely runneth over with more credit-card offers, sweepstakes entries, and supermarket fliers. That's why we get so much junk e-mail: It's essentially free to send.
I think this argument is actually very weak: Neither U.S. Postal Service nor FedEx nor Deutsche Post nor <insert your favourite postal service here> will be able to deliver mail for free in the next 100 years, because that would be financial suicide. Sending a letter from $A to $B requires a lot of manpower on the side of the postal service. And this manpower needs to be paid regularly with the money coming from the postage.
On the other side, the costs for processing eletronic mail are only a fraction of the costs for delivering "real" mail: It takes nearly no manpower to operate a sufficient machine, hardware (even the good one) is relatively cheap and bandwidth isn't a problem as well. And if there is really no way for email providers to get their money from anywhere else but email, they still have the option of embedding text ads in the mails, which are processed by their infrastructure. Yahoo, MSN and several others have shown that this works out very well.
So if you are looking for arguments, why email should be made non-free (*), then please choose them carefully: Comparing email to letters does not work out at any rate.
(*) For me this sounds like a fundamentally bad idea. But that's yet another story to be told.