A study into spam has blamed it for the production of more than 33bn kilowatt-hours of energy every year, enough to power more than 2.4m homes.
The Carbon Footprint of e-mail Spam report estimated that 62 trillion spam emails are sent globally every year.
This amounted to emissions of more than 17 million tons of CO2, the research by climate consultants ICF International and anti-virus firm McAfee found.
Searching for legitimate e-mails and deleting spam used some 80% of energy.
The study found that the average business user generates 131kg of CO2 every year, of which 22% is related to spam.
ICF say that spam filtering would reduce unwanted spam by 75%, the equivalent to taking 2.3 million cars off the road.
However, the ICF goes on to say that while spam filtering is effective in reducing energy waste, fighting it at the source is far better.
The report highlights the case of McColo , a US web hosting firm that had ties to spammers. The day after it was taken offline by its two internet service providers, global spam volume fell by 70%.
Although the respite was only temporary, McAfee said the "day without spam amounted to talking 2.2 million cars off the road" and that tackling spam should be part of the campaign to reduce carbon emissions.
Richi Jennings - an independent spam analyst who helped produce the report - told the BBC that the figures were based on the extra energy use spent dealing with spam.