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.