In order to balance this, and to avoid this becoming an impediment to personal use of the Internet, the fee should be small and should cancel if two parties are communicating repeatedly (in both directions).
The basic idea is to charge a fee for the transmission of email. This fee goes, not to the carrier, but to the recipient. This helps cover the cost of reading and handling the mail.
In normal communications, this fee will cancel out between two parties, or a group of people. In legitimate advertising, the advertiser is willing to pay a reasonable amount to get you to read their ads, and, in fact, benefits from the fact that you will want to read their mail partly because of the fee. Spammers, however, would quickly find that sending 10 million emails, at ten cents (say) for each non-response, is financially impossible.