Our online spam filter will only accept connections from known IP addresses.
We have a 3300 with 4 WAN lines, 1 of which is not static.

Is there any way that we can force SMTP traffic out of one of the static lines by default?

e.g. All SMTP from LAN server 192.168.1.6 is forced to use WAN2 and thus appears at the remote end as coming from 86.1.1.1