Microsoft is offering a $250,000 reward for help in catching the hackers behind the Rustock botnet.

At the peak of its activity, in April last year, the Rustock botnet had infected 2.5 million computers and was sending out 43 billion spam emails a day. This slowly shrunk in size until March this year when Microsoft went to US courts to claim control of the Rustock server, after the botnet started sending out spam featuring Microsoft's logo, a process which the tech giant said was infringing its copyright.

"This reward offer stems from Microsoft's recognition that the Rustock botnet is responsible for a number of criminal activities and serves to underscore our commitment to tracking down those behind it," said Richard Boscovich, Senior Attorney at Microsoft's Digital Crimes Unit in a blog.

"While the primary goal for our legal and technical operation has been to stop and disrupt the threat that Rustock has posed for everyone affected by it, we also believe the Rustock bot-herders should be held accountable for their actions."

Microsoft said that while the Rustock infection base had been cut in half since the takedown, "there are still hundreds of thousands of infected computers around the world yet to be cleaned of the botnet malware".

"We will continue to follow this case wherever it leads us and remain committed to working with our partners around the world to help people regain control of their Rustock-infected computers."

In June this year, Microsoft placed quarter-page notifications in two Russian newspapers, to notify unnamed defendants in the legal suit and give them an opportunity to make their case.

The legal formality was required as part of the ongoing lawsuit. However, no-one related associated with Rustock would step forward. Microsoft believes Rustock's operators are located in Russia.