Routing SMTP traffic through specific alias defined for a specific gateway

kindly i would like to know how to route smtp traffic from lan to specific alias on a specific gateway
 
currently iam on SFOS 18.0.0 EAP3
 
i have a rule on rules and policies that scan all smtp traffic
 
and another NAT rule to specific translate address
 
but still smtp traffic donot go from the defined alias on the gateway
 
 
it goes from the gateway ip even in routing i donot have the option to route through specific alias
 
only specific gateway is available