Spam levels in the second half of March are back to those seen before the removal of the McColo hosting service in November 2008, according to Google.
McColo was notorious for providing so-called "bulletproof" services to cybercriminals, who wanted to keep their servers running no matter what.
When McColo was knocked offline, it had a serious effect on the world's junk email. Spam levels dropped by half the instant the company's upstream Internet service providers - Global Crossing and Hurricane Electric - refused to service the ISP, effectively unplugging McColo from the Internet.
"Spammers have clearly rallied following the McColo takedown and overall spam volume growth during Q1 2009 was the strongest it's been since early 2008, increasing an average of 1.2 percent per day," wrote Google spokeswoman Amanda Kleha in a blog posting.
In the first quarter of last year, spam grew at 1 percent per day, which was a record at the time.
Data on another spam measurement site, Spamcop, also shows that spam has bounced back.
Spammers may be getting smarter, too, Kleha said. They seem to be building more robust botnets to send out their unwanted mail and also appear to be taking steps to avoid making their ISPs the kind of obvious target that McColo was.
"They have been building new botnets, like Conficker - which is undoubtedly designed to be difficult to take down," said Richard Cox, CIO of anti-spam organisation Spamhaus, in an email interview. He agreed with Google's conclusion that spam had now returned to pre-McColo levels.
"Spammers continue to prove their resilience," Kleha said. "They're clearly here to stay."