IRCX (Internet Relay Chat eXtensions) is an extension to the IRC protocol, developed by Microsoft.
IRCX defines ways to use SASL authentication to authenticate securely to the server, channel properties/metadata, multilingual support that can be queried using the enhanced "LISTX" command (to find a channel in your language), an additional user level (so there are three levels: owners, hosts, and voices), specific IRC operator levels, and full support for UTF-8 (in nicknames, channel names, and so on). IRCX is fully backwards compatible with IRC; the new features are downgraded to something a standard IRC client can see (and UTF-8 nicknames are converted to hexadecimal).
IRCX was originally supported on Microsoft Exchange 5.5 (in place of the old Microsoft Chat protocol, which is a binary protocol) and a module was available for Microsoft Exchange 2000.
Microsoft has since stopped distributing software that supports IRCX, and morphed its protocol into the protocol used on the MSN Chat network, which was not standardized or openly available for use (however, its usage was very similar to IRCX and therefore most IRCX clients were able to connect to MSN Chat without much modification). MSN Chat closed on 16 October 2006.
Microsoft started to put IRCX through a standardisation process with the IETF by publishing 4 Internet Drafts of their protocol, but the standard was never ratified. Because of this, every IRCX implementation bases itself on these draft papers, of which version 4 is the latest.