Microsoft Corp. is releasing its long-delayed instant messaging software today and, in a twist likely to annoy rival America Online Inc., the software will let people communicate not only with other people who use it but also with people using AOL's Instant Messenger.

Deanna Sanford, lead product manager for Microsoft's consumer and commerce group, said the company's MSN Messenger Service software will be integrated with Microsoft's free Hotmail e-mail service on the Web and its Outlook Express 5 e-mail software. It will launch from a button in Microsoft's Web browser, Internet Explorer 5.

All of that Microsoft has long planned, and its online competitors fully expected. But Microsoft has gone further, Sanford said, by designing its new messenger so that users will be able to freely send messages to and receive messages from AOL users -- and even automatically import their AOL "Buddy List."

It is unclear whether AOL users can add Microsoft instant messengers to their buddy lists or detect their presence online.

An AOL spokeswoman said yesterday that Microsoft gave AOL no advance notice of its plans to connect with AOL's instant messenger, so the Dulles-based online giant hasn't prepared a response.

"I guess we should be flattered," AOL spokeswoman Ann Brackbill said. "But if there is unauthorized interoperability between the two, it seems surprising that Microsoft would do that."

Some analysts saw this move as a gambit by Microsoft to undermine AOL's dominance of the messaging world. If the Microsoft software takes off, it could eventually look more attractive than AOL's -- Microsoft users could talk to everyone, detect users' presence online and become "invisible" to their contacts through the software's new elaborate privacy features. AOL users, meanwhile, could talk to MSN users, but may have trouble detecting their online presence or adding them to their buddy lists.

AOL has more than 40 million people registered to its instant messaging services, called Buddy Lists inside AOL's proprietary service and AOL Instant Messenger on the World Wide Web. Together, those 40 million AOL customers send more than 430 million messages each day, nearly as many as the 500 million letters sent daily through the U.S. Postal Service.

Instant messaging software lets people detect when friends are online and send them text messages that are delivered in pop-up boxes much faster than e-mail. AOL has another, more sophisticated messenger called ICQ that about 38 million people have downloaded.