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Exchange Routing
Port 691 is used by Microsoft Exchange Server for routing email messages within and between Exchange servers in an organization. It plays a vital role in facilitating message transport and ensuring efficient communication across the messaging infrastructure..
Port 691 is primarily associated with Microsoft Exchange Server's routing service. This service manages the transfer and delivery of emails between Exchange servers, leveraging routing groups and connectors within the Exchange environment. The routing service uses port 691 over TCP to communicate message-related commands and status updates, facilitating reliable and organized mail flow.
In earlier versions of Exchange (notably Exchange 2000 and 2003), the routing engine used a message transfer agent (MTA) to exchange routing updates and transport mail between servers. While newer versions have shifted towards different transport mechanisms, knowledge of port 691 remains important when dealing with legacy systems or during migration processes.
Effective operation of port 691 helps maintain the internal communication fabric of an Exchange organization, supporting directory lookups, message categorization, and routing logic. Administrators should monitor this port chiefly in environments where legacy Exchange routing components are still active.