The ASTA (Anti-Spam Technical Alliance), which comprises AOL, Earthlink, Microsoft and Yahoo, issued the document, here in PDF form. It endorses in a general sense the two technical approaches being developed in the industry.
IP-based authentication validates that purported senders of e-mail are in fact legitimate senders for their domains. Spammers often utilize weaknesses in current Internet e-mail standards to make mail appear to come from a different domain than that of the actual sender.
Both Microsoft and independent researchers have worked on systems using this approach and have developed a consolidated standard, called Sender ID, based on SPF (Sender Policy Framework) and Microsofts own Caller ID. Microsoft says this proposal will be submitted to the IETF within the next several weeks.
Authentication based on content signing, which uses cryptographic techniques and digital signatures, also ensures that the message is sent by legitimate senders from the domain of the purported sender. Yahoo has developed a specification for content signing of e-mail called Domain Keys and submitted it to the IETF in May.
The ASTA document also endorses certain best practices for actors in the e-mail industry. Among the recommendations to ISPs are to limit rates on e-mail by users and otherwise limit the use of port 25, the Internet port for e-mail sending; to close all open relays, which are e-mail servers open for all to use; and to detect and shut down zombied client computers, which are computers taken over by Trojan horses and worms and used to send spam.
The alliance recommends that legitimate bulk e-mail senders comply with the law by not using forged e-mail headers, harvesting e-mail addresses through SMTP or Web pages without user consent or using misleading subject lines. It also urges that users install and make use of anti-spam, anti-virus and other security software.
All four members of the alliance plan to experiment with both authentication approaches, and all have implemented SPF to some extent. Microsoft and Earthlink have implemented Caller ID as well.
None would commit to a time for implementing Domain Keys or another content signing specification, but Microsoft hopes to make recommendations on it by the end of the year based on ongoing testing.
In a conference call, alliance members emphasized that authentication itself is not an anti-spam technique, but that it is a tool to be used by filtering, reputation and accreditation systems to block spam and identify legitimate e-mail to let through.