Microsoft has filed lawsuits against 13 unnamed senders of spam email messages through botnets.

The actual charges were filed in August, but Microsoft has only now unveiled details about the case.

A botnet is a collection of hacked computers (referred to as zombie PCs) that are at the disposal of the botnet operator. They rent the machines out to send spam, host illegal websites such as child pornography sites or launch distributed denial of service attacks.

Earlier this year the software developer intentionally infected a computer with an internet worm which resulted in it being made part of a botnet. The firm then quarantined the machine and monitored it over a period of 20 days.

During this time the zombie was contacted 5m times by spammers and was instructed to send out a total of 18m spam messages, advertising 13,000 individual websites.

No spam was actually sent and the computer worm was also contained, Microsoft said. The data collected in this probe resulted in the 13 lawsuits.

Spammers are increasingly moving to zombie networks as internet providers and enterprises have cracked down on open relays. An open relay is a mail server that is configured to allow computers outside its network to send email messages.

Microsoft unveiled the legal assault at the kick-off of an international campaign to promote consumer action in protecting their PCs and stop the spread of zombie PCs. The initiative includes the UK's Get Safe online programme. 

Microsoft in the past has filed lawsuits against senders of spam emails, but until now has never gone after spammers using botnets.

The legal assault comes weeks after authorities in The Netherlands arrested three individuals who are accused of operating a botnet of more than 1.5m hacked PCs.