Microsoft Corp. has made its Sender ID framework for email authentication license free.

As of Monday, the technology is available through the software maker's Open Specification Promise, which makes Microsoft technology available to commercial, open source and academic developers for free. Sender ID is the third area of technology Microsoft has released under the OSP.

Email and Internet service providers have been reluctant to license Sender ID from Microsoft. As a result, it has had less adoption in the industry then rival email-authentication technologies, such as DomainKeys Identified Mail, which is favored by Yahoo Inc., the largest email provider; and network supplier Cisco Systems Inc., experts say.

"By putting Sender ID under the Open Specification Promise, our goal is to put those questions to rest and advance interoperable efforts for online safety worldwide," Brian Arbogast, corporate vice president of the Windows Live Platform Development Group at Microsoft, said in a statement.

Sender ID uses Microsoft's Purported Responsible Address method for checking the headers with a message body to verify the legitimacy of the domain name from which an email is sent. Sender ID also supports the Sender Policy Framework Classic method, which looks only at envelope headers outside the message body; but the authentication system generally implies the PRA method.

In September, Microsoft made its Web services specifications the first technology area available under the OSP. Earlier this month, the company added its Virtual Hard Disk (VHD) Image Format specifications.