Taking a quick glance at your inbox and/or junk mailbox, I suspect you'll see plenty of email messages that you'll be deleting immediately. According to computer security company McAfee, there are about 62 trillion of those messages sent each year, and they consume enough electricity (33 billion kilowatt hours of electricity) to power 2.4 million homes.
McAfee issued a report about this, but there seem to be a fair number of wholes in the theories. It's difficult to discern what the computers would be doing if they weren't filtering spam. Email servers do not typically power themselves off just because they're receiving 10,000 messages instead of 4,000. Of course, it would be beneficial if the messages weren't sent to begin with, and that could save some power ... but what would those CPUs doing instead of sending spam?
It's not surprising that McAfee would want to point this out and sell its security software to people. However, if anything, that would create more power being wasted, versus just deleting the spam emails by hand. Pretty much everyone (except spammers) agrees that spam is bad, but this calculation is a bit of a stretch. I have thousands of spam messages sitting in random email accounts, and there's not much I can do about it. Maybe there's a way to have a hamster power my email filters, but I don't think that's possible right now.