bind9 is in FreeBSD base system, you don't need to install anything from ports to set it up.
So basically you have one mail gateway that has to route mail for different domains to different guest systems with IP addresses from private address space.
For that you have to set up virtual transports on the gateway. Check out transport_maps and relay_domains tables.
You'll just need to add smtp:YOURGUESTIP (or YOURGUESTHOSTNAME) as transports for your domains and mail will be routed to that IP/HOSTNAME.
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Fhtagn nagh Yog-Sothoth
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