According to research by climate consultants ICF International and anti-virus firm McAfee, email spam uses more than 33bn kilowatt-hours of energy every year.
(This is enough to power more than 2.4m homes, and amounts to emissions of more than 17 million tons of CO2. Not just that, but searching for legitimate e-mails and deleting spam uses 80% of the energy, so of the 131kg of CO2 created by the average business user 22% of it is related to spam. According to the same report spam filtering would reduce unwanted spam by 75%, the equivalent to taking 2.3 million cars off the road. I've never heard of wanted spam.)