People are being sent fewer spam emails than at any time in the past 12 years, according to security firm Symantec.
It said that fewer than 50% of emails which it scanned during June were junk – the lowest percentage it had seen in over a decade.
The fall suggests that cyber-thieves are looking to other ways to make money from people going online.
The security firm said legal action against the criminal networks that pipe spam to people had helped to cut junk.
However, it said the fall in spam had been offset by a rise in the amount of malware being made.
In June only 49.7% of the billions of messages monitored by Symantec were spam, it said in its latest monthly threat report. While the amount of spam being sent had been declining for some time, this was the first time since 2003 that it had dipped below 50%, said Ben Nahorney in the report.
Since the report was issued, spam levels have continued to decline further. On 14 July, of the 25 billion messages monitored by Symantec only 46.4% were junk.
It said the decline was caused by action against botnets, networks of hijacked computers that have historically been the route through which spam has been sent.