Make sure that the IPv4 routing is enabled:
sysctl net.ipv4.ip_forward; grep forward /etc/sysctl.conf
It is configured in /etc/sysctl.conf.
Make sure that all mail related traffic of the cluster nodes is routed through the load balancer host (otherwise, clients receive replies bypassing NAT and reject them). This is true when the load balancer is also an IP-router for the cluster nodes. If not, configure the load balancer's address as default route for nodes.