Microsoft's ploy to drum up business for its spam-filtering technology will probably be successful!
That said, from my experience, the number of people depending upon e-mail forwarding must be considerable -- from an e-mail volume standpoint, perhaps staggering!
Being an alumnus of two universities, having a primary e-mail address at work, a wireless e-mail address, and an e-mail address required by my broadband provider -- all of which forward to the same Exchange inbox, I expect that Microsoft's actions will result in a dramatic drop in the reliability of the existing spam detection technology -- especially for the mobile professional who also subscribes to a large number of listservs which might also be impacted.
It took months for my employer's spam filters (which send me a quarantined list of two-dozen every day -- for my review) to mature to the point that the number of false positives are down to about three per week! If successful, Microsoft's plan will likely introduce large numbers of false positives to the workplace e-mail environment without significantly improving the spam problem for hundreds of millions of casual users of e-mail.
Spam really represents two distinct problems. One is the "legitimate" mass market operation who used to send you "junk mail" until they discovered that sending e-mail was cheaper than bulk-rate postage. These folks, while a considerable annoyance, do at least have a legitimate reason for contacting you. They want to tell you why their product is better than the next guy's product. The other is the unscrupulous e-mailer who wants to steal your identity, sell you porn, or get you to buy some fake product (be it a pharmaceutical or a sexual aid).
There are laws to protect you from both of these types of unwanted e-mail. The problem is that we don't know who the sender really is -- or what jurisdiction governs their activities. One more anti-spam tool which generates false positives will not solve this problem!
The solution can only be an underlying redesign of Internet e-mail, developed under the auspices of a sanctioning body (such as IEEE), which guarantees that sender are who they say they are and that the jurisdictions of senders can be determined with certainty.
[Editor's note: David Berlind looks at the slow progress toward an anti-spam standard.]