McAfee/ICF released The Carbon Footprint of Email Spam Report in 2009. Some excerpts from the report's "Key Findings":

• An estimated worldwide total of 62 trillion spam emails were sent in 2008
• Globally, annual spam energy use totals 33 billion kilowatt-hours (KWh), or 33 terawatt hours (TWh). That’s equivalent to the electricity used in 2.4 million homes in the United States, with the same GHG emissions as 3.1 million passenger cars using two billion United States gallons of gasoline
• The average GHG emission associated with a single spam message is 0.3 grams of CO2. That’s like driving three feet (one meter) in equivalent emissions, but when multiplied by the annual volume of spam, it’s like driving around the Earth 1.6 million times
• A year’s email at a typical medium-size business uses 50,000 KWh; more than one fifth of that annual use can be associated with spam
• Much of the energy consumption associated with spam (80 percent) comes from end-users deleting spam and searching for legitimate email (false positives)

The report urges two actions to reduce spam's energy/carbon footprint.

One is to use spam filtering. "If every inbox were protected by a state-of-the-art spam filter, organizations and individuals could reduce today’s spam energy by approximately 75 percent or 25 TWh per year. That’s equivalent to taking 2.3 million cars off the road ...Spam filtering accounts for just 16 percent of spam- related energy use"

The other is to put major spammers out of business. "Filtering spam is beneficial, but fighting spam at the source is even better. When McColo, a major source of online spam, was taken offline in late 2008, the energy saved in the ensuing lull—before spammers rebuilt their sending capacity — equated to taking 2.2 million cars off the road". Disabling spammers clearly has the most leverage, but permenant solutions elude us. The report makes passing reference to "proposals ranging from large email providers banding together to implement sender authentication systems to pay-to-send models."

Update 2011.01.05
The Radicati group estimates, "In 2010, the typical corporate user sends and receives about 110 messages daily. Roughly 18% of emails received is spam, comprising both actual spam and “graymail” (i.e. unwanted newsletters, alerts, etc.)." Using 0.3 grams of CO2e per, we get ~6 grams/user/day. Assuming 250 business days a year, each corporate user emits ~1.5 Kg CO2e/year dealing with email. Radicati projects 850 million corporate email accounts worldwide for 2012, yielding a total annual CO2e spam/greymail footprint of over 1200 gigatonnes.