Microsoft has added its anti-spam SenderID technology to a list of open specs.

The email specification detects when an email address is being spoofed to send spam, and can now be used as a basis for new technology by anyone under Microsoft's Open Specification Promise (OSP), said Jason Matusow, senior director of inter-operability.

Through the OSP, published in September, Microsoft promised it would not take any patent-enforcement action against people that want to use its specifications on a list of Web services technologies. Sender ID has now been added to that list.

Microsoft last week opened access to its virtualisation technology, the Virtual Hard Disk specification, in the same way.

Sender ID was developed by Microsoft, SendMail and other companies as a type of Caller ID system for email, so recipients of messages could keep track of where the email originated to ensure they are legitimate and not spam. The Internet Engineering Task Force is currently working on the development of the Sender ID specification, the first draft of which was released in June 2004.

Sender ID allows companies to attach information to an Internet domain that tells email recipients what addresses are authorised to send mail from it. In addition to protecting users against spoofed email, Sender ID also allows message recipients to identify email addresses that send spam and "assign a reputation based on their behaviour," said John Scarrow, general manager of anti-spam efforts at Microsoft.