Several users of Microsoft's MSN Messenger have reported receiving an instant message from the AOL Instant Messenger service (AIM), warning them that they are accessing the AOL network using "unauthorized software."
The message reads: "Please be advised that you are currently accessing the AOL network using unauthorized software," and provides a link to where AIM software can be downloaded.
"As part of our blocking strategies, we are sending instant messages to MSN users which basically informs them that they are accessing AOL servers on an unauthorized client," said AOL spokeswoman Tricia Primrose.
The battle between Microsoft and AOL flared almost immediately after the software giant launched its MSN Messenger service two weeks ago. One of the prominent features of the new service was its ability to connect with the popular AIM service. AOL, however, countered by blocking the rival service from accessing its "Buddy Lists."
The two companies have since fired volleys back and forth, with MSN offering "fixes" to punch holes in AOL's blockades and AOL erecting new barriers.
Industry analysts have noted how crucial it is for rival IM services to be able to connect to AIM given its 40 million members. Others, including Yahoo Messenger, tout their ability to connect to AIM.
The sparring also has drawn other companies into the slugfest. AOL and Apple Computer said they would develop instant messaging products that allow "seamless" communication between Mac users and AIM.
AOL also formed an advisory group that includes its chief technology officer Marc Andreessen, Apple's interim chief executive Steve Jobs, Sun Microsystems' Bill Joy, and RealNetworks' Rob Glaser, among others.
Last week, Microsoft sent a letter to AOL asking its chief executive Steve Case to open the online giant's proprietary systems.