Its possible that certain outgoing connections be routed specifically through the first ISP link at ISP Redundancy Load Sharing Mode
edit the $FWDIR/lib/table.def as follows
By changing it to: no_misp_services_ports = { <500, 17>, <259, 17>, <80,6>};, (where <25,6> stands for SMTP (port 25), TCP (IP protocol 6)), all outgoing SMTP traffic would go through the first ISP link.
Also some tips
Show the currently defined ISP links
#cpstat fw
Test ISP Redundancy by administratively bringing down/up thelink
# fw isp_link ISP-1 down
# fw isp_link ISP-1 up
more advanced commands will be on next release of SmartSPLAT