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This figure comes from a study by Ferris Research, which found that UK firms receive more spam than businesses in France, Germany, Italy and China. The research group calculated that spam filtering software could have a return on investment of between 30% and 220%, or higher for companies that receive large volumes of junk e-mail.
But Ferris warned that suppliers' claims about the effectiveness of spam filtering software can be exaggerated.
Leading suppliers claim their software is capable of removing 95% or more of spam automatically, with false positive rates of three legitimate messages filtered out as spam for every 100,000 incoming messages.
But in practice, false positive rates can be much higher, ranging from 0.01% (equivalent to one false positive every 20 weeks) and 1% (equivalent to five false positives a week).
Ferris said desktop anti-spam products are a false economy, costing organisations an average of £113 per user per year, compared with £69 per user for server-based systems.
This is because desktop spam filtering software costs more to buy, install and maintain than server-based software.
Leaving staff to control their own junk e-mail costs an average of £374 per user per year, Ferris said.
There may be a business case for doing this in some parts of the world, such as China and India, where labour costs and spam volumes are low.