"One way this might be implemented is by charging senders a small fee per e-mail sent, often referred to as a "Sender Bond." It might be close to free for an advertiser to send a single e-mail message to a single recipient, but sending that same e-mail to 1000 recipients would cost him 1000 times as much. A 2002 experiment with this kind of usage-based e-mail pricing found that it caused senders to spend more effort targeting their messages to recipients who would find them relevant, thus shifting the cost of deciding whether a given e-mail message is relevant from the recipient to the sender (Kraut 2002)."
I've read about a very similar phenomena over at http://en.wikipedia.org/wiki/Attention_economy.
"One way this might be implemented is by charging senders a small fee per e-mail sent, often referred to as a "Sender Bond." It might be close to free for an advertiser to send a single e-mail message to a single recipient, but sending that same e-mail to 1000 recipients would cost him 1000 times as much. A 2002 experiment with this kind of usage-based e-mail pricing found that it caused senders to spend more effort targeting their messages to recipients who would find them relevant, thus shifting the cost of deciding whether a given e-mail message is relevant from the recipient to the sender (Kraut 2002)."