Strange issue we have here. First time I've come across this.
Question: Is it possible to use an access-list on a NAT IP address on a Cisco router? For example, say we have our internal mail server 192.168.1.5 and it's NATed to the outside on port 25 say to 18.104.22.168. Is there a way to apply an access list to this external IP so that only certain outside users can get to this server using port 25??