To achieve redundancy and load balance for send connectors, is usefull create one sender connector. Create one Send Connector and add two anti-spam servers into smart hosts list.
Create one Send Connector and add two smart hosts list. It can achieve redundancy and load balance. The both smart hosts work together, each one take half of tasks. Anyone of them breaks down, another one will take all task.
Exchange Server 2007 routing load balancing and fault tolerance