Microsoft said that Live Communications Server 2005 will come with optional components that will allow enterprise users connect directly with users of Yahoo! Messenger and AOL Instant Messenger.

The move is remarkable as both AOL and Yahoo are on board, all three firms having spent several years avoiding cooperation when it comes to compatible IM. Microsoft said the deals are the next step in making EIM as widespread and beneficial as e-mail.

Microsoft is paying for the privilege of allowing its buyers to connect to the networks of its competitors. Financial terms and pricing for the software have not yet been disclosed. Microsoft expects LCS 2005 to ship in the fourth quarter this year.

It’s not quite interoperability. It’s more of a one-way street. Users behind an LCS 2005 server with the options installed will be able to see presence information of users on all three networks, whereas other users will have to use each network separately.

Asked about the potential for interoperability, a Yahoo spokesperson said: We have no plans yet, but this is definitely a step in the right direction. We’ve always been a proponent of opening up the IM community.

FaceTime Communications Inc’s CTO Jonathan Christensen, who was once on the LCS team at Microsoft, said: This is not interoperability, this is interconnection. The question now is whether interconnection will lead to interoperability.

LCS, part of Office, is a presence server, using the SIP and SIMPLE protocols. Other Office applications, such as Outlook, will be able to leverage this presence data. The add-ins will translate the proprietary AIM and Yahoo protocols to SIMPLE.

LCS 2005 will allow users to connect externally, like when they are at home, and get the same level of connectivity without the need for a virtual private network, Microsoft said. Federated presence between LSC-using companies will also be possible.