Hotmail customers will play an integral part in the company's fight against junk e-mail, Bill Gates has said in an open letter, while also dismissing the idea of generating revenue from spammers by imposing a charge for sending e-mail.

"Monetary charges would be inappropriate and contrary to the fundamental purpose of the Internet as an extremely efficient and inexpensive medium for communications," he wrote.

The letter, entitled "Preserving and Enhancing the Benefits of Email - A Progress Report", reviewed the company's work and outlined future plans in the fields of filtering, sender authentication and other preventive measures, plus collaboration with regulators and law-enforcers.

It is the latest in a series of e-mail messages in which Gates and other Microsoft executives address customers, partners and computer users on issues involving technology, public policy and the IT industry. A year ago, Gates also addressed the problem of spam in a message entitled "Toward a Spam-Free Future".

Spamming is becoming more difficult and less lucrative, Gates said in Monday's e-mail. For Microsoft customers, the situation is improving thanks to the introduction of Microsoft's SmartScreen spam filter, which it deployed on its Hotmail Web-based e-mail service six months ago. In his message a year ago, Gates said filters on servers at MSN and Hotmail blocked 2.4 billion messages a day. This year, the figure is up to around 3.0 billion messages a day, or 95 percent of incoming spam, Gates wrote. That still means Hotmail is letting through an average of one junk message per user per day.

SmartScreen is also available as a free download for users of Exchange Server 2003, Gates said. Over the next year Microsoft will upgrade the filter, "drawing upon millions of messages that hundreds of thousands of volunteer MSN Hotmail customers have contributed and marked as either spam or non-spam," he said.

In addition to improving filtering technology, Gates wrote, Microsoft must: Make sure e-mail from regularly correspondents is delivered automatically; allow unknown senders to offer proof that they're not spammers; and prevent spam from entering networks.

The company is working with other members of the Anti-Spam Technical Alliance to combat e-mail forgery by adopting systems for authenticating e-mail senders, Gates wrote. Microsoft has also had a hand in developing the specification for one such system, Sender ID.

Other areas in which collaborative efforts are bearing fruit include law enforcement. Following the help that Microsoft gave US federal agencies that filed joint civil and criminal actions against a group of spammers in April, he said, the company filed a further 17 lawsuits in June, bringing the total number of anti-spam lawsuits it launched to over 90 worldwide.

On the technology front, the company is looking at ways to increase the computational difficulty of sending e-mail by giving the sender a puzzle to solve before a message is accepted for delivery, he wrote. Solving one such puzzle to send a personal message would not be difficult; solving hundreds of thousands of them to complete a spamming run would pose problems. But Gates ruled out the possibility of introducing systems to charge for the sending of messages.

Microsoft is studying other ways to keep spam off corporate networks. Microsoft Exchange Edge Services will bring together a number of techniques to stop spam from getting in, or e-mail addresses from leaking out, Gates said.

The company also plans to offer a self-updating filter, he wrote. That product was also in the pipeline when Gates wrote his letter last year, indicating that the fight against spam is going to be a long one.