Customers of Microsoft Corp.'s Hotmail service play an integral part in the company's fight against junk e-mail, Chairman and Chief Software Architect Bill Gates said in an open letter last week. He also dismissed 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, and collaboration with regulators and law enforcement officials. Spamming is becoming more difficult and less lucrative, Gates said. 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, he wrote in the e-mail.