McColo

McColo was a US-based web hosting service provider[2] that was, for a long time, the source of the majority of spam-sending activities for the entire world.

[3] In late 2008, the company was shut down by two upstream providers, Global Crossing and Hurricane Electric, because a significant amount of malware and botnets had been trafficking from the McColo servers.

[7] McColo was one of the leading players in the so-called "bulletproof hosting" market — ISPs that will allow servers to remain online regardless of complaints.

According to Ars Technica and other sources, upstream ISPs Global Crossing and Hurricane Electric terminated service when contacted by Brian Krebs and The Washington Post’s Security Fix blog,[8][9] but multiple reports had been published by organizations including SecureWorks, FireEye and ThreatExpert, all naming McColo as the host for much of the world's botnet traffic.

Spamhaus.org reportedly finds roughly 1.5 million computers infected with either Srizbi or Rustock sending spam in an average week.