One router, two IP address - can I port forward to 2 servers

I have eight (I think) fixed IP addresses (a block?). I only use one.

I have one Fritz!Box router where I port forward mail 'n' stuff to a local server.

But say, for legitimate but complex reasons, I need to run another mail server on another server on my LAN. How - using the tools I have - can I route mail for @thisdomain to ThisServer using port forwarding while also sending mail for @thatserver to ThatServer.

In Fritz!Box I can add a "Public IPv4 Subnet", it says
"If your internet service provider supplies you with a public IPv4 subnet, you can enter it here."

So, if I have the DNS setup for thisserver.tld to point to ThisServer I can also do the same for ThatServer.

But the Fritz!Box instructions appear to say ThatServer won't be on my LAN anymore. "Network devices that are assigned a public IP address no longer obtain their IP settings from the FRITZ!Box. Therefore, they cannot access other devices in the FRITZ!Box home network or the FRITZ!Box user interface." It's not the same as the post forwarding I have set up already which goes to a 'server' I can access locally on my LAN.

Am I Very Confused? Or have I got this Spot On? Or maybe there's a Better Way?

Choice

Multi NAT - no idea if your router supports this. Exact model number needed for someone to figure it out

OR

Setup router with NON NAT and route one public IP to the mail server and then another public IP to an Ethernet router behind which you have your local network.

In reply to a post by MrSaffron:

Multi NAT - no idea if your router supports this. Exact model number needed for someone to figure it out

FritzBox 7490https://en.avm.de/service/fritzbox/fritzbox-7490/ove...

Setup router with NON NAT and route one public IP to the mail server and then another public IP to an Ethernet router behind which you have your local network.

I think that might be what the FB instructions are saying, in a slightly different way. My second IP address is non-nat, traffic is routed directly to one server, the other IP routes via the FB's router and so can access the LAN.

you should be able to do it in port forwarding.
e.g.:
port 25 on wan ip a goes to this server
port 25 on wan ip b goes to that server

that's how I've set up multiple servers on the same router if they need the same port

In reply to a post by danielhyde:

port 25 on wan ip a goes to this server
port 25 on wan ip b goes to that server

Yes, what I thought and imagined but that (according to my reading of the FritzBox instructions) opens the server on WAN B to the internet without the protection of NAT or port forwarding.

I might look for a less hard way to go about this. I might be asking the wrong questions and I only need it for a few hours.