Yahoo's email service has begun to use the company's DomainKeys authentication technology, almost a year after Yahoo first unveiled this antispam project.

"This is a really big milestone for us, because with this authentication solution in place, we're increasing protection from ID (identification) theft and phishing," said Terrell Karlsten, a Yahoo spokeswoman.

DomainKeys attempts to tackle a practice called spoofing, in which a spammer changes an email message's header information to deceive recipients into believing the message was sent by a known and trusted personal acquaintance or legitimate business, such as a bank or credit card company.

With spoofing, spammers try to increase the chances that recipients will open and reply to their email messages. Very often, spammers' ultimate goal with spoofing is to trick recipients into revealing confidential information, such as social security numbers, bank account information and passwords – scams often referred to as "phishing".

Yahoo's DomainKeys uses cryptography to let receiving email systems validate that an incoming message originated from a user authorised to send email for the domain stated in the header. The receiving email system uses a public key to validate a private-key signature in the incoming message.

Other providers of internet and email services, such as Microsoft and AOL are also involved in individual and collaborative initiatives to combat spam. For example, EarthLink is announcing Monday that it will begin testing DomainKeys.

In related web mail news, Yahoo is announcing that it is increasing the in-box storage space for users of its free web mail service from 100MB to 250MB, as a result of user feedback. This puts Yahoo on par with Microsoft's free Hotmail web mail service.

However, both Yahoo and Microsoft still lag far from the 1GB Google is offering with its Gmail free web mail service, which is still in testing phase and available only by invitation to a limited amount of users.

It was Google's Gmail announcement in April that jolted web mail providers to increase storage sizes almost across the board. Yahoo used to provide 4MB for its free web mail users, while Microsoft provided 2MB, before they both increased their in-box storage in recent months.