One tech columnist pointed out that much spam is sent from zombie PCs anyway, so spammers can use hacked PCs to send spam, and have the zombie's owners foot the bill.
Quote:

There is also potential for abuse in the postage stamp model. What the pay-per-email model doesn't necessarily stop is corporate networks being hijacked to send spam: in the vast majority of cases, spammers don't even use their own computers to send mail. Right now, many company computers end up being hacked and used to send thousands of messages. If your firm pays 0.1p for every email it sends, and one machine gets turned into a zombie that sends 5 million emails, it would sting you for £5,000. I can't think of anybody who'd fancy that.


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