What do AOL, Google, Microsoft, and Yahoo have in common, apart from the letter "o"? Among several possible answers, spam is certainly one. The billion email inboxes managed by the four companies get far too much spam, including plenty of phishing attacks.

In an effort to find a solution to their common problem, the four companies have been working with a startup called Agari to implement the Agari Email Trust Fabric, a email authentication layer that limits the effectiveness of phishing.

Agari's system can't stop phishing entirely because it's a social engineering attack: Phishing works by duping the recipient of a phishing message into clicking on a malicious link that appears to be legitimate. But the company claims it can eliminate a particularly effective attack vector: The misuse of trusted email domains.

The Agari Email Trust Fabric prevents malicious email senders from spoofing legitimate email domains within its set of protected mailboxes. It does this through a cloud service built using SPF and DKIM, two established domain authentication technologies.

[ 2011 was a big year for Google. Find out more in Google's Daring Dozen: 12 Big Bets In 2011. ]

Citing an RSA study, Patrick Peterson, CEO of Agari, said in a phone interview that phishing costs companies $1 billion annually.

Financial services companies, in particular, suffer from phishing, both in terms of fraud losses and diminished customer trust. But perhaps not much longer: With Agari's technology, Peterson said that some large clients are seeing 50 million phishing messages blocked per day.

"That is a massive win for them," he said, noting that Agari's system can stop 100% of phishing messages that rely on spoofing the domains of its customers. That's not every phishing attempt--deceptively misspelled domains that look like legitimate domains, for example, may not be caught--but it's a start.

Peterson says Agari can help companies understand how their online identity is being used and can help them enforce policy controls.

Google product manager Adam Dawes, in a statement, observed that Google has been working with various email authentication standards since 2004. While coordinated authentication between sender and receiver has historically proved to be a challenge, he suggests that Agari's approach can help.

"Agari's approach simplifies the authentication process for large email senders, helps them communicate clear policies to receivers like Gmail, and creates higher communications value for their email transactions," he said.

Agari's system also gathers massive amounts of data for its customers. With the help of analytical tools, customers can see what's going on in their email channel and they can share their findings with authorities or ISPs to help shut down email abuse.

Peterson, however, insists that Agari doesn't see its customers' data, and won't become a magnet for authorities seeking access to email users' data. "We think [our system] draws the line quite nicely between privacy concerns and actually providing some actionable threat intelligence," he said.

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