You can have the discourse receiver bind to an ipv6 address (wrap the address in square brackets) and the external postfix act as a relay. It’s a little fiddly, as you have to make sure that the external postfix doesn’t bind to the ipv6 address, but it sounds like you can figure it out. I just did this to be able to run 15 mark receivers run on a single host (though only one is running now).