What Is the Categorizer?

What Is the Categorizer?
The Categorizer is a component of the Exchange 2000 transport process. When a message is submitted to the transport process, the Categorizer uses the header information on the message to query Active Directory for information about how and where the message must be delivered. For example, from a Simple Mail Transfer Protocol (SMTP) address such as someone@example.com, the Categorizer identifies the Exchange 2000 server that contains the users mailbox and determines how to route the message to the server. The Categorizer also expands distribution lists and applies per-user limits to messages.

The Categorizer relies on DSAccess for the list of Active Directory servers that it should use for lookups; however, like DSProxy, it uses its own mechanism to read information from Active Directory after it has the server list.