I have one Exchange 2010 server with 2 upstream email gateways for antispam/antivirus filtering. Outbound email can go through either gateway - both have send connectors. The primary gateway's send connector is configured with a cost of 1, and the secondary with a cost of 3 so that mail goes through the primary when possible. However when the primary fails mail backs up in it's Exchange queue. Exchange does not seem to understand that the gateway is down, and refuses to requeue the mail to the secondary send connector.
The result is that mail backs up until it expires, and it bounces back to the sender - even though there is a completely functional alternate send connector. How do I configure Exchange to requeue mail through the secondary gateway when the primary's queue is filling up?