Microsoft thinks it knows best how to fight the problem of spam:

If your e-mail does not have a Sender ID, Microsoft wants to junk your message. Sometime around November, Hotmail and MSN will flag as potential spam those messages that do not have the tag to verify the sender, Craig Spiezle, a director in the technology care and safety group at the software maker said Wednesday. The move is meant to spur adoption of Sender ID, he said. Sender ID is a specification for verifying the authenticity of e-mail by ensuring the validity of the server from which the e-mail came. While the purpose of curbing junk mail may be laudable, the debate on how to stop the tide of junk mail is still ongoing. According to Microsoft, up to 90 percent of e-mail is spam.

While I'm glad Microsoft wants to help in the fight against spam, any attempt to force widespread adoption of Sender ID will cause more headaches for you and me than for spammers. Sender ID tags are inserted into email messages by your outgoing mail server. In the case of most consumers, this is a server provided by your ISP. When your message arrives at its destination, the recipient's incoming mail server looks at the address from which you sent the message and the Sender ID of your outgoing mail server. It then looks at a list of approved domains for which your outgoing mail server can send messages to see if your email provider is listed. If there's a match, your message gets through. If not, it's marked as spam. This is all well and good if your email provider is included in your ISP's list, but there's no guarantee that it will be. If you have multiple email addresses from multiple email providers, and you use a mail client like Outlook Express or Thunderbird to send messages from all of them, you're probably only using your ISP's outgoing mail server to send everything. Thus, if EarthLink is your ISP, but you have a NetZero email address, any mail you send with the NetZero account is actually sent via EarthLink's outgoing mail server. The message never even passes through a NetZero server (unless the recipient has a NetZero address). This discrepancy exists because most email providers who also sell Internet service only make their outgoing mail servers accessible to users who are also Internet service customers. Hence, if you connect to the Internet via EarthLink, NetZero won't allow you to connect to its outgoing server, even though you have one of the company's email addresses. Meanwhile, EarthLink will allow you to send messages from just about any of your email accounts because you are actually connected to the Internet through its company's service. What this means is that, unless "netzero.net" (or whatever your email provider's domain happens to be) is listed as an approved domain for EarthLink's outgoing mail server (and it probably isn't), your email address and your outgoing mail server's address will not match. As a result, your email will be considered spam. But even if your mail provider is approved, your ISP's outgoing mail server still has to support Sender IDs for the process to actually work correctly. Most do not, meaning that, come November, the vast majority of legitimate email sent to Hotmail accounts will automatically be headed for the spam folder. Of course, Bill Gates isn't going to read that email you sent him anyway, so why should he care that it was automatically forwarded into a rubbish bin? [CNET News.com] Microsoft pushes spam-filtering technology (via GigaLaw)