Spam filters were catching significantly less spam according to independent security tests of 20 anti-spam products.
The VirusBulletin comparative tests found in many cases products missed twice as much spam as they did in the same test conducted in recent months.
Virus Bulletin anti-spam test director Martijn Grooten said it was a “worrying trend", possibly attributable to better evasion tactics employed by spammers.
"There have been many news stories highlighting a global decline in spam in recent months, but if spam filter performances decline too, the situation for the end-user doesn’t improve at all," Grooten said.
“It may be a sign that they are increasingly using compromised legitimate systems to send their messages.”
Libra Esva performed best in the tests, blocking 99.97 per cent of spam without false positives.
It was the only product to take out the VBSpam+ award which requires products block at least 99.5 per cent of spam and no legitimate email.