Thank you both for your replies, most helpful. I did wonder about the routing table and whether that would need amending. I looked at PBF for another situation but the requirement went away so never got to test it out. As far as routing goes, on our single virtual router, I have the two default 0.0.0.0/0 routes for each WAN interface with their associated next hop IP address and the metric is lower for ethernet1/5, which explains why the traffic is hitting that interface along with regular internet access, despite the new NAT rule. What I didn't understand is how the server out on the internet does show the desired outbound IP hitting their firewall, despite it being the wrong IP for the wrong interface. Is it spoofing that IP somehow?? The traffic is definitely heading out of the 'wrong' interface though. Sounds like PBF may actually be the quicker and easier way forward for this? I could actually set it up for that all internet traffic from these specific internal addresses follows the alternative WAN interface. It doesn't need to be just for that one destination internet IP.
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