Messenger for Mac offers two types of communication services- a personal account and a corporate account. A personal account works with the MSN® Messenger service on the Microsoft Passport Network. Contacts that you add to your personal account will include friends and family members. A corporate account uses the Microsoft Office Live Communications Server service and can include contacts who use other instant messaging services, such as AOL Instant Messenger (AIM), Yahoo Messenger, and iChat users who are signed in with AOL accounts.This new version is supposed to exhibit an improved compatibility with firewall and proxy.
However, audio and video are still not supported for the Mac, so alternative instant messengers still have a bright future.
You can download Microsoft Messenger for Mac 5.0 from Microsoft website.
