Spammers have regrouped and are finding ways to send more junk mail despite recent efforts by security experts.

Spam levels dropped by almost half when rogue ISP McColo was taken offline in November. But some new botnets and even older ones are churning out more spam.

"At the current rates, we'll be back at those pre-McColo takedown levels probably within the next three to five weeks," said Adam Swidler, senior product marketing manager for Google Message Security, also known as Postini.

Google said Monday it has seen a 156% increase in spam since McColo went offline. McColo hosted the so-called command-and-control servers for botnets that are used to instruct PCs to send spam. The botnets included Rustock, Srizbi, Pushdo/Cutwail, Mega-D and Gheg.

McColo's takedown for the most part killed off the Srizbi botnet, which was blamed for sending a large proportion of the world's spam. But other botnets -- which are essentially legions of hacked computers configured to send spam -- are picking up the slack.

Mega-D, also known as Ozdok, is comprised of at least 660,000 PCs, according to MessageLabs, an e-mail security outfit now owned by Symantec. On average, PCs infected with Mega-D send out an astounding 589,402 messages per day, or around 409 per minute. All told, Mega-D is sending out 38 billion messages per day.

According to MessageLabs' latest figures released Monday, 74.6% of all e-mail was spam this month, a 4.9% increase over December. Percentages of spam can vary by vendor depending on the pool of PCs using their services, which are used to collect statistics on spam.

"We've seen a steady increase over the last two months," said Paul Wood, MessageLabs Intelligence Analyst with Symantec.

MessageLabs saw spam drop to around 58 percent of all e-mail when McColo went down, but rising to around 69% in December, Wood said.

Spammers are also changing their tactics to ensure their messages are not blocked, said Richard Cox, CIO for the antispam organization Spamhaus.

When a computer is infected with code used to send spam, it sets up a mail server on the PC, which proceeds to pump out spam directly onto the Internet. But if that computer is noticed sending spam, it is added to a block list of end-user IP (Internet Protocol) address ranges that shouldn't be sending unauthenticated mail.

The IDG News Service is a Network World affiliate.