Connectors are chosen from among those that are not currently retrying (waiting on an Open Retry timer). This approach, known as preemptive rerouting, is used so that the MTA does not attempt to route a message to a connector that failed the last time it tried to establish a connection.
Suppose that you have a Site Connector and a Dynamic RAS Connector with a higher cost, and the LAN is down. A message is first routed to the Site Connector because it has a lower cost. When the connection fails because the LAN is down, the message is rerouted through the Dynamic RAS Connector. Although the cost is higher, this preemptive rerouting is more likely to achieve timely delivery of the message.
Any new message that comes in before the Open Retry timer has expired on the Site Connector skips the Site Connector and is routed first to the Dynamic RAS Connector.