Three companies representing the world's largest block of e-mail users joined hands Monday to fight spam as they increasingly weigh the cost of preventing unsolicited e-mail from clogging inboxes, networks and computer memory space.

America Online Inc. (AOL), Microsoft Corp. (MSFT) and Yahoo! Inc. (YHOO) said they would work together to devise ways to block unidentified messages, stop spammers from creating fraudulent e-mail accounts and work more closely with law enforcement to reign in the practice.

Users fed up with spam, or e-mail touting everything from get-rick-quick schemes to porn and sex toys, are calling on their e-mail service providers to stop the flood of unwelcome advertisements from spammers, or spam senders.

"Spammers are public enemy No. 1," said Nicholas Graham, spokesman at America Online Inc., the Internet unit of AOL Time Warner.

According to a report by Forrester Research, North American online users said more than one-third of their e-mail is junk mail, with the average consumer getting bombarded with 110 unwanted e-mails every week.

America Online has 35 million subscribers, Microsoft's MSN Internet service has over 9 million subscribers, plus millions more with accounts on their Hotmail free e-mail service. Yahoo does not disclose its e-mail user data.

Taking on spammers is nothing new for these companies, which already employ a variety of technologies to filter out unsolicited messages and thwart their efforts to register for free e-mail accounts that they use as a launchpad for spam.

"It's not just about using technology, but also building a coalition for more collaboration and more cooperation," said Brian Arbogast, corporate vice president at Microsoft for delivering authentication and mobile service at MSN.

For example, Microsoft, America Online and Yahoo would share the techniques they use to identify e-mail whose author and origins have been concealed.

America Online, the world's largest Internet service provider, filed five separate lawsuits against more than a dozen individuals and companies two weeks ago for sending about 1 billion unsolicited messages, hawking everything from pornography and steroids to college degrees and mortgages.

Microsoft and EarthLink Inc., a smaller Internet access provider, have also pursued legal action.

Other companies, such as anti-virus vendor Trend Micro Inc. and private start-up Qurb Inc., are offering software that users can buy and install to block spam.

Despite all these efforts, the companies admit that it may be nearly impossible to completely eradicate unsolicited e-mail, as spammers work furiously to devise new ways of circumventing the virtual roadblocks thrown in their way.

"Spam is like the cold virus of the Internet," said Yahoo's Graham, "Its going to exist in some form or another, like taxes or bad weather."