The massive spam reduction caused by McColo's closure was quickly back to normal within days

That fact has left some researchers a little surprised that the latest decline in spam has lasted as long as it has.

"The volumes are still way down," said Matt Sergeant, senior anti-spam technologist at MessageLabs. "

"For now, bots that are searching for a C&C master are more visible, so FireEye is reaching out to the victims and notifying them of how to disconnect themselves from the botnet," Lin said.