hi Alex,
what DNS server are the hosts currently using for name resolution?
say in your case, your DNS server is 10.10.10.10/24 and all your clients are seeing the router(say 10.10.10.1/24) as their gateway.
what you can do here is to set-up your router itself as the primary DNS server where you create a mapping for the mail server against the private IP address of the server. you would need to edit your DHCP config for this and set the DNS server to be 10.10.10.1
And on the router, set your DNS server(10.10.10.10) as the forwarding address on the router.
So basically this is how this is going to work:
The clients are going to see the router as their primary DNS server. All
DNS query requests will first be sent to the router. the router will
resolve for the mail server to it's INTERNAL IP address. For all other queries( for eg hosts
trying to reach google.com) will be forwarded by the router to the 10.10.10.10 DNS server.
here is how you can configure your router for this:
1. Ip dns server
2. Ip host 10.10.20.1 -------------mapped the domain name to the IP address
3. Ip name-server 10.10.10.10 ------------set the previously configured DNS as the forwarding address
4. Ip domain look-up
i have tried this solution once and it worked fine. let me know if it works for you.