When a user sends a message to a particular distribution list (DL), the Exchange 2000 Server machine receives that message and queries Active Directory (AD) to determine whether the group has a designated expansion server. If an expansion server is assigned, the Exchange 2000 server forwards the message to that server. If no expansion server is assigned, the Exchange server expands the DL locally. In Exchange 2000, the Message Categorizer handles DL enumeration, whereas in Exchange Server 5.5, the Message Transfer Agent (MTA) handles DL expansion. During the expansion, or enumeration, process, the Exchange 2000 server Message Categorizer contacts a Global Catalog (GC—from the working GC list that DSAccess created) and asks the GC for a list of members in that DL (the Windows 2000 group). If the DL is a global group, the GC replies with a list of users from the Member attribute. If the DL is a universal group, the GC looks at the Member attribute of the group and sends what it finds. Keep in mind, however, that if your environment consists of more than one domain and you’re sending to a global group, the Member attribute may be null, meaning that the group is legitimate, just empty. You don’t receive a nondelivery report (NDR) if you send a message to an empty group. For this reason, universal groups are better than global groups for email distribution for Exchange 2000 in an AD environment.