A common misconception is that outbound email can be load-balanced for Exchange simply by provisioning two equal cost Send Connectors, either using DNS to route directly or routing via a smart host for each Send Connector.
When the cost of the Send Connectors and the proximity to their source servers are the same, Exchange will simply choose the one with the alphanumerically lower connector name, and will not load balance the outgoing email across both connections.
The correct solution is to deploy a single Send Connector with multiple smart hosts.
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