AOL is the latest and largest company to back away from Microsoft's Sender ID software, designed to cut spam by identifying the source of the email.

The move is particularly embarrassing for Redmond as AOL was the first in a list of companies it said last week had voiced support for the technology.

AOL will continue to use the Sender Policy Framework (SPF) with which Microsoft's technology is combined in Sender ID.

"SPF is the 'low-hanging fruit' in the authentication debate and, given the momentum and common ground with the SPF protocol, is the first logical step in the journey to combat spam," AOL said in a cautiously worded statement.

"AOL remains committed to testing authentication technology in the real-world environment of large scale ISPs."

Earlier this week, the Internet Engineering Task Force (IETF) turned down Microsoft's technical contribution to the proposed standard, and expressed concern at the company's use of patents.

Open source companies the Apache Software Foundation and Debian had already stated that Microsoft's royalty-free patent licence agreement for the technology was incompatible with open source.

A Microsoft spokesman put a brave face on the announcement. "AOL's decision to conduct only 'mail from' checks as outlined in the original SPF proposal reflects exactly the kind of flexibility and room for choice provided by the IETF's revised Sender ID proposal," he said.

"What is encouraging about AOL's announcement is that it will join us in publishing both records, and we continue to recommend that all mail senders do the same."

Speaking of the IETF response to Microsoft's technical contribution, the spokesman added: "While we would have preferred a single technical mechanism as the standard, we believe that the IETF's proposal to allow multiple scopes in the protocol is a reasonable approach to provide additional choice and flexibility.

"We will continue our collaboration with industry stakeholders to help move this important authentication protocol forward."