Wasn't Windows Messenger before MSN Messenger:
Version 2.5.1 is an update to version 2.5, which is a mandatory update to version 2.1 that is available for users running Mac OS 9.2.2.
Version 3.5 for Mac OS X revisions older than 10.2.8
Version 4.0.1 for Mac OS X revisions 10.2.8 and newer
Version 5.1.1 for Mac OS X revisions 10.3 and newer
Version 6.0.3 for Mac OS X revisions 10.3.9 and newer
Version 7.0.0 for Mac OS X 10.4.9 or newer was released on April 29, 2008.
Version 7.0.1 for Mac OS X 10.4 or later includes minor bug fixes.
Version 7.0.2 for Mac OS X 10.4 or later: improves overall quality and conversations with Microsoft Office Communicator 2007 R2.
Version 8.0.0 beta for Mac OS X 10.5 or later, for Intel Macs only: Official Beta released on March, 24th 2010. Adds A/V conferencing between Mac OS X users and Windows Live Messenger 2009 users.
Version 8.0.0 for Mac OSX 10.5 or later, for Intel Macs only: Released with Microsoft Office 2011 for Mac in October 2010 and bears a visual similarity to previous Windows counterparts. It features somewhat more stable functional voice and video calls.