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The functionality of the message transfer agent (MTA) has changed significantly in Exchange 2000 Server. This article provides a brief overview of these changes.
RoutingThe Exchange 2000 MTA, unlike earlier Exchange Server MTAs, does not perform routing functions. It calls the routing engine, which uses Link State tables for more intelligent routing. These tables are built by using the Link State Algorithm, which is based on Dijkstra's 1959 algorithm. This algorithm has been used extensively on the Internet for many years in the form of Open Shortest Path First (OSPF) routers. The Link State Algorithm propagates the state of the messaging system in almost real time to all servers in the organization, which allows for more intelligent routing than the routing that is available in earlier (GWART-based) versions of Exchange Server. One advantage of using Link State tables is that it eliminates the constant re-routing that occurs in earlier versions of the MTA when a destination link or server is not working. The Exchange 2000 MTA makes calls to the routing engine by using the Mtaroute.dll file.
Distribution List ExpansionThe Exchange 2000 MTA does not expand distribution lists (DLs). The Message Categorizer now performs this function. To expand DLs, earlier version MTAs use the Home-MTA property to define DL expansion servers. Exchange 2000 uses the Ms-Exch-Expansion-Server-Name property.
Message Fan-Out Procedure to Appropriate QueuesThe Exchange 2000 MTA does not expand DLs. The Message Categorizer now performs this function. To expand DLs, earlier version MTAs use the HomeMTA attribute to define DL expansion servers. Exchange 2000 uses the MsExchExpansionServerName attribute.
Content ConversionContent conversion between X.400 P2/22 and MDBEF (Microsoft Exchange format) is still performed the same way in Exchange 2000 as it was in Exchange Server 5.5. The MTA still performs this task.