krou writes "A new study entitled "The Carbon Footprint of Spam" (registration required) by ICF International for McAfee claims that spam uses around 33 billion kilowatt hours of energy annually, which is approximately enough to power 2.4 million US homes, or approximately 3.1 million cars, for a year. They calculated that the average CO2 emission for a spam email is around 0.3 grams. Interestingly, the majority of energy usage (around 4/5ths) comes from users deleting spam, and searching for legitimate emails. They also claim that "An individual company can find that one fifth of the energy budget of its email system is wasted on spam". One of the reports authors, Richi Jennings, writes on his blog that "spam filtering actually saves an incredible amount of energy": "Imagine if every inbox were protected by a state-of-the-art spam filter. We could save about 75% of the spam energy used today--25 TWh per year; that's like taking 2.3 million cars off the road.""