In Designing a Messaging Topology, you were introduced to three components of a messaging topology: Messaging Server, Directory Server, and clients. This section will describe other components in a basic messaging topology.
Messaging Server. Houses and maintains user mailboxes; it can also be a server that contains just the MTA portion of Messaging Server as described in Internet-facing MTA and MTA Relay.
Client. Accesses messaging services from Messaging Server (often through the Messaging Multiplexor).
Directory Server. Used by Messaging Server for name and alias lookup. Direct LDAP lookup determines where messages should be routed.
Messaging Multiplexor. Connects clients to the appropriate Messaging Server for retrieving messages.
MTA Relay. The inbound MTA routes incoming messages to valid addresses in the appropriate Messaging Server. The outgoing MTA accepts outgoing messages from clients, queries LDAP to find out where to send the message, then sends it off to the appropriate server or out across the firewall to the Internet. Typically, a Messaging Server host is set up to perform this function.
DNS Server. Resolves server names into IP addresses to allow messages to be routed to their proper address in the network.
Firewall. Restricts Internet access of your internal site. You might even have a firewall between departments in your organization.