Reverse-spamming
“Another factor that would mitigate spam if POW tokens have value: there would be a profit motive for people to set up massive quantities of fake e-mail accounts to harvest POW tokens from spam. They’d essentially be reverse-spamming the spammers with automated mailboxes that collect their POW and don’t read the message. The ratio of fake mailboxes to real people could become too high for spam to be cost effective.”
Satoshi Nakamoto (Jan 25, 2009) http://satoshi.nakamotoinstitute.org/emails/cryptography/18/