Spam levels are starting to rise again now that some notorious botnets are back online, according to Symantec.

Across the globe, spam levels had dropped 75 percent after Internet service provider McColo was shut down earlier this month. Botnet owners apparently have found a new ISP and bandwidth provider, as spam volumes have doubled since last week, reports eWEEK.

One botnet that seems to be rearing its ugly head again is Srizbi. Thought to be defunct, it is picking up speed again, according to The Washington Post. Security company FireEye reports that a number of rescue domains were registered Tuesday evening, directing at least 50,000 infected machines to receive new instructions and malicious software updates from servers in Estonia.