The message categoriser is a plug-in to the advanced queuing engine. It is essentially a collection of event sinks that perform advanced address resolution on every message that travels through the advanced queuing engine. These messages might be destined for the local Web Storage System, a remote host through the message transfer agent (MTA), or a remote host through SMTP. The message categoriser also handles the expansion of distribution lists. The message categoriser is turned off by default in Windows 2000; Exchange 2000 activates it and enhances it with a set of categoriser event sinks, known as Phatcat, in Phatcat.dll.