Microsoft's MSN Messenger will let users send notes and messages to users of AOL's Instant Messenger program and other popular programs, including its own Hotmail Web-based e-mail system. AOL dominates the instant messaging space, with both its AOL program and ICQ, the service offered by Mirabilis, which AOL acquired last year. Together, the two programs have 63 million registered users, according to AOL. Microsoft said that the MSN Messenger service will be available at midnight July 22 at messenger.msn.com. "Just as consumers expect different telephones to work with one another, so they should expect different instant messaging services to talk to each other," Brad Chase, vice president of the Consumer and Commerce Group at Microsoft, said in a release. "MSN Messenger Service is the first messaging application to offer the interoperability people are demanding." Microsoft also said today that it is working with a group of companies to develop a set of standards for instant messaging. AOL was not listed among those companies. AOL did not immediately return calls for comment. But the company does have plans to beef up its own services. It's ICQ unit said today that it is working on a free e-mail program for ICQ users, and has already signed up 400,000 users. The company did not say how soon the service, developed with Critical Path, would be ready.