Google has integrated the anti-spam standard first introduced by Yahoo, that of Domain Keys, into the test of its Gmail service

Google's free email service, Gmail, has begun using anti-spam technology supported heavily by archrival Yahoo.

Messages from Gmail sent on Monday indicated that they were encoded with DomainKeys technology, as evidenced by logs in the message headers. When an email header from a Gmail message was opened, a code reading "DomainKey-Signature" appeared.

DomainKeys is a technology backed by Yahoo that tries to cross-check email messages to verify their origination. The idea is to thwart email "spoofers," which are spam messages that pretend to be from legitimate Internet addresses. These spoofed emails often pass through spam filters and fool recipients into opening the messages.

"We are encouraged by the adoption of DomainKeys and believe it is a key step in furthering the industry's fight against spam and email forgery," Yahoo spokeswoman Mary Osako said. "Spam is an industry issue, and we believe collaboration with the Internet community is vital in solving the authentication issue."

A representative from Google was not immediately available for comment.

DomainKeys is a proposed system that attaches encrypted digital tags to each email, then compares them to a public database of legitimate Internet addresses. Once the message arrives, the database and message keys need to match in order to pass into an email in-box.

Yahoo is not the only Web giant to propose an anti-spam system. Microsoft has been pushing Sender ID, which verifies the authenticity of the sender's "@" address by checking the message's numeric Web address. Sender ID blends Microsoft's proprietary "Caller ID for Email" with another technology, called Sender Policy Framework, or SPF.

America Online has put its weight behind SPF and has begun installing the system into its email servers. The online giant recently stopped testing SenderID after standards bodies and the open-source community pulled their support and criticised Microsoft for keeping the technology under a shroud of secrecy. AOL is still testing DomainKeys, which works in tandem with SPF or SenderID.

All three companies are standing behind separate technologies and have each filed proposals to the Internet Engineering Task Force for standardisation.

For Google, the addition of anti-spam technology is the latest step by the search giant to beef up its email service. The company in April began testing Gmail and launched it with 1GB of mail storage, sending shock waves across the industry. Soon after, Yahoo upped its email storage to 100MB and Microsoft boosted Hotmail to 250MB.

Gmail's adoption of DomainKeys also could add legitimacy for Yahoo's pursuit of standardisation.

CNET News.com's Stefanie Olsen contributed to this report.

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