Botnets are now responsible for distributing 87.9% of all spam, with approximately 151 billion unsolicited messages each day being distributed by compromised computers, say researchers at MessageLabs Intelligence.

The largest botnet now appears to be Rustock, with an estimated 1.3 million to 1.9 million compromised computers in its control. MessageLabs says botnet technology has evolved significantly since the end of 2008 and the most recent closures now have a seemingly limited impact on the botnet activity, with downtime and outages lasting for only a few hours, rather than weeks or months as before.

In other analysis, MessageLabs found that overall, 80% of domains being blocked as malicious for serving up malware are in fact compromised, legitimate Web sites. These sites tend to take longer to identify (and to get cleaned up) than sites that are set up purely to distribute malware.

Malicious sites are generally taken down within 38 days of registration; compromised sites can take up to 138 days to be cleaned up.