By Mark Ward
Technology correspondent, BBC News website
More than 95% of e-mail is junk, be it spam, error messages or viruses, report mail monitoring firms.
Some home computers are compromised by e-mail viruses
Analysis of the contents of millions of e-mails has revealed that less than 4% is legitimate traffic.
Further work has shown that most of this junk mail is originating on hijacked home computers.
E-mail security firm Return Path said 99% of the computers it monitors that send mail have been taken over by spammers or virus writers.
Return Path reached its estimate by calculating a "reputation score" for the 20 million net addresses of those machines.
The score was derived by analyzing the e-mail traffic sent through those addresses, the number of complaints filed about that address, and if the owner of that address responds to complaints.
The vast majority of these net addresses were not good net citizens, said George Bilbrey, spokesman for Return Path.
Only 1% of net addresses could be regarded as legitimate sources of mail.
The rest, said Mr Bilbrey, were hijacked computers, or bots, used by spammers and other net criminals to send e-mail.
Typically these home computers are compromised by viruses sent in e-mail messages or by worms that trawl the net looking for vulnerable machines.
Matt Peachey, regional director for IronPort, which monitors about a quarter of all mail sent across the net, said its research revealed that about 80% of e-mail came from compromised hosts.
Though, he said there were some cases such as with Polish Telecom where that percentage was likely to be much higher.
The vast majority of the 170,000 net addresses sending e-mail from Polish Telecom were probably zombies, he said.
Ironport's analysis of the e-mail traffic sent to large corporations reveals that more and more of the messages are junk that is not worth reading.
Mr Peachey said statistics for one of its customers show that from 20 June to 20 July only 4% of the messages received were legitimate.
Of the rest, he said, 70% were spam, 11% were bounces or error messages and 9% was viruses.
On average, said Mr Peachey, only about 10% of e-mail was now real.
The news comes as anti-virus firm Sophos reveals the list of nations where most spam starts its journey across the net.
Top of the list is the US as, according to Sophos, 23.2% of spam originates there.
This is probably due to the fact that broadband has become very popular in the US and has led many people leave inadequately protected PCs connected to the net all the time.
Graham Cluley, senior technology consultant at Sophos, said in the previous two quarters the amount of spam originating in the US had fallen but this decline had now stopped.
"Perhaps the reality is that the statistics can't be reduced any further unless US home users take action to secure their computers and put a halt to the zombie PC problem," he said.
In the number two position was China which sends out 20% of junk e-mail.
Although China has a low percentage of net users in its total population, more than 123 million Chinese people regularly go online and more than half of those do so via broadband.
Significantly the US and China occupy the number one and two spots in counts of total numbers of net users.
Next in the list come South Korea (7.5%), France (5.2%) and Spain (4.8%). The UK was tenth with 1.8%.