According to Consumer Reports' 2006 State of the Net survey, which collected 2000 respondents with Internet access, over the past two years U.S. users lost A$10.5 billion (US$8 billion) as a result of online scams including viruses, spyware and phishing schemes.

Peter Stewart, CEO of antispam vendor TotalBlock, said those affected would save billions if blocking was used instead of other filters.

"If all those users had used antispam software that relies on challenge-response (blocking) techniques rather than the usual filtering technology, billions of dollars would have been saved, because blocking results in zero spam," Stewart said.

"Challenge-response works by blocking all machine generated e-mail [by] building a list of acceptable incoming e-mail senders, using an address book as well as automatically replying to senders who are not on the allowed list [with] a simple action that requires human intervention to add the sender to the list."

However, research firm Hydrasight managing director Michael Warrilow said this exclusion reduces blocking's relevance.