For years the Necurs botnet has distributed junk mail and malware for many different groups of cyber-thieves.
But the amount of malicious traffic emerging from Necurs has now dwindled to almost nothing.
It is not clear what has caused the slowdown and whether traffic will return to previously high levels.
One of the first signs of the disruption was seen earlier this month when email messages spreading the Dridex banking trojan and Locky ransomware caught by security firms dried up.
