The Federal Trade Commission's shutdown of internet service provider Pricewert has not had the same crushing effect on spam as the McColo closure last year.

Security companies reported some drop in spam levels and botnet activity, but they are quickly recovering. Overall spam and botnet activity is now approaching the levels present before yesterday's shutdown.

"So far our guys haven't seen anything different," said Graham Cluley, senior technology consultant for Sophos. "McColo was the last of the dinosaurs. Now a multi-headed hydra, it's getting much harder to knock out the botnets."

A large botnet known as Cutwail, which is estimated to be behind more than a third of all spam, did see a temporary loss of activity. Cutwail dropped to almost nothing after the shutdown, but is now back on the rise.

"The McColo shutdown really allowed spammers to prepare for this latest shutdown," said Matt Sergeant, senior anti-spam technologist at MessageLabs.

"For now, we will see spam levels lower than usual, but we expected the swift comeback of Cutwail. The spammers learned that they can't put all their eggs in one basket and need to have backup command and control."