Criminals have been conducting complex, targeted e-mail attacks against employees at more than 100 e-mail service providers (ESPs) over the past several months in a bid to hijack computers at companies that market directly to customers of some of the world’s largest corporations, anti-spam experts warn.

The attacks are a textbook example of how organized thieves can abuse trust relationships between companies to access important resources that are then recycled in future attacks. According to multiple sources, the so-called “spear phishing” attacks in this fraud campaign arrived as virus-laden e-mails addressing ESP employees by name, and many cases included the name of the ESP in the body of the message.