The four major ISPs – Microsoft, AOL, Yahoo and Earthlink – are fighting the spam problem head on, as witnessed by their recent suits against what they consider to be the most notorious spammers.

In addition to the legal route, however, all four are currently testing new technologies aimed at identifying and stopping the increasing wave of spam crossing their networks. Since your marketing messages and other critical e-mails must successfully traverse the same technologies, today’s Web publishers would do well to monitor their activities. Here’s a quick overview of the three main spam-fighting contenders:

* Caller ID for E-mail. Microsoft recently announced a new spam-fighting technology it calls Caller ID for E-mail. It is known as an anti-spoofing strategy in that it verifies the domain from which a message is sent in an effort to stop spammers who spoof the origination addresses of legitimate e-mailers. Brightmail announced it is partnering with Microsoft to combine Caller ID with its own Reputation Service. Together, the two will not only ensure the message comes from a trusted domain but also provide a scale to measure the sender's reputation for sending legitimate e-mail vs. spam. To read the technical specification for Caller ID or to send your comments about it to Microsoft, go here.

* DomainKeys. This is technology offered by Yahoo that is currently being tested by Yahoo and Sendmail. It uses a form of cryptography to generate a unique signature verifying an e-mail sender’s identity. Considered among the more complex of the spam-fighting tools, it goes a step further than Caller ID in that places added authentication and verification at the signature level. To learn more, visit Yahoo here.

* Sender Policy Framework (SPF). SPF is currently being tested by AOL and is considered an anti-forgery tool. Like Caller ID, it requires that domain owners publish the IP addresses from which they send e-mail. Then, mail transfer agents, such as Sendmail, match the client IP address with the domain the message is coming from. If the client IP address doesn't match the published IP addresses for the domains, the message is rejected before it ever gets to the inbox. SPF is being championed by a group called SMTP+SPF. For more information, visit their site here.

Overall, the new technologies are a cut above current spam filters in that they rely on true authentication schemes. In order for their e-mail to be carried, senders must agree to be authenticated, a procedure to which few spammers would be willing to submit, since it would make them easily and readily identifiable.