Levels of spam have fallen by almost 50% since August 2010, suggest figures.
Figures compiled by security firm Symantec show that the amount of junk e-mail messages flowing around the net has dropped 47% in three months.
Kaspersky Labs noted a similar fall from July to September, when spam levels fell to 81.1% of all e-mails
The decline was put down to the arrests of those behind spam-sending botnets, and intelligence work that saw other spamming systems shut down.
In the last few months security firms have scored several notable successes against gangs that own and operate botnets – collections of hijacked home computers.
The vast majority of spam or junk mail is routed through these hijacked machines.
One of the biggest successes was against the Pushdo or Cutwail botnet, which had been in operation since 2007 and was thought to be sending about 10% of global spam.
An international operation co-ordinated by the security firm LastLine managed to get 20 of the 30 servers controlled by the group shut down. The servers were turned off with the help of the internet service providers unwittingly found to be hosting them.
As a result, many of the “drone” PCs in the huge botnet used to send e-mail were cut off and no longer relayed the junk messages.