MAIL server behind 2 wan ips

Hello! I have 2 wan load balancing using ppc method (wan ip 1.1.1.1 and 2.2.2.2). I have mail server ip 10.10.10.10 behind the router. How to force outgoing traffic from mail server to go only through ip 2.2.2.2. Is it just src nat rule?

IP firewall mangle, do mark routing, check the wiki for more info