Microsoft Exchange Server MTA Routing

The Microsoft Exchange Server MTA can send mail externally to:

Microsoft Exchange MTAs in the same site, by using remote procedure calls (RPCs).
Microsoft Exchange MTAs in a different site, through the Site Connector.
Microsoft Exchange MTAs in a different site, through the Dynamic RAS Connector.
Remote X.400 MTAs, through an X.400 Connector such as X.25, Transport Control Protocol/Internet Protocol (TCP/IP), or TP4.
Gateways such as the Microsoft Mail Connector or Internet Mail Service.

Associated with each connector is an address space (or spaces), cost, and possibly a connected site (or sites). These associations are defined by using the Microsoft Exchange Server Administrator program, and are subsequently used to generate the Gateway Routing Table (GWART). An enterprise-wide GWART is created in each site by the Routing Information Daemon (RID), which accesses local information and replicated site GWARTs to build a consolidated local GWART.

A mail message can be either a P1 message, probe, or report. Each responsible recipient of a message is routed individually. Before routing a recipient, the MTA obtains as much address information for the recipient as possible. For more information, see the "Name Resolution" section. If the recipient is local to the current MTA's address space, local routing takes place. If not, remote routing takes place. This process involves routing (through the GWART), selection, and load balancing for each recipient to obtain the routing destination.