A common misconception is that outbound email can be load-balanced for Exchange simply by provisioning two equal cost Send Connectors.
Well, this will not work. In this case, Exchange will simply choose the Send Connector with the alphanumerically lower connector name, and will not load balance the outgoing email across both connections, even if the cost of both Send Connector is the same.
The correct way is to deploy a single Send Connector with multiple smart hosts pointing to many smtp servers.
That way load balancing and fault tolerance are accomplished even for outgoing email traffic.