Hey
I set up NixOS mail server some days ago at Netcup a German Hosting provider. I ordered a server from them. I used nixos-infect and made my own custom script for the networking to be working else it would not connect to the network and ssh keys were the only available way to access the machine so i had to reset is a lot when i trying to get it work and made a lot of mistakes.
I then setup a Mailserver.nix file. and followed the documentation for simple nixos mail server which you can find if you click on the link i provided earlier. next i setup My users in the mailserver.nix file.
I used nix-shell to generate the password hashes.
I will leave some link below to the relevant sources: