Yahoo and Cisco have teamed up in an effort to reduce the amount of junk email reaching users' inboxes.

The firms have announced a specification called DomainKeys Identified Mail (DKIM) that they hope will become a web standard. DKIM combines Yahoo's DomainKeys and Cisco's Identified Internet Mail authentication technologies.

Yahoo's DomainKeys uses the Domain Name System (DNS) to check and authorise an email's signature against its original source. If it appears suspicious or forged, the email can be blocked or quarantined. Cisco's Identified Internet Mail works in a similar way, but it can use the Key Registration Server (KRS) system or DNS to verify the sender.

But although DomainKeys has support from some large customers - namely Gmail and Earthlink, as well as Yahoo itself - Cisco's system has not gained much attention. This could be due to the availability of rival email authentication systems like SenderID.

Paul Woods of email security specialist MessageLabs said, "Until there is a definite standard [for email filtering], it is going to be very difficult to tackle these types of threats." He added that the ability to check who has registered domain keys may offer little protection if spammers can register signatures as "legitimate businesses".