how to force protocol to exit on precise IP on multiIP WAN

Another question:

I have setup in which I have multiple WAN links (which are properly mangled, nat-ed).
Every WAN link have more then one public IP address.

Now, what I want is to make sure that when I try to send SMTP traffic from my mail server, it goes from particular WAN link and particular IP adress from that WAN link to the outside world.

Any ideas on how to do that?

In my current setup it simply goes on my main internet link and I do not see how can I control that. (WAN links are configured to be backup of one another, not as load balancing links)

Mark the SMTP connection.
Put a route mark on the connection.
Add a route to the routing table using the route mark.
Add a srcNAT rule for SMTP from that server IP

Hope that helps

Nick.