{
  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";

  outputs = { nixpkgs, ... }:
    {
      nixosConfigurations.spanmail = nixpkgs.lib.nixosSystem {
        system = "x86_64-linux";
        modules = [
          ./configuration.nix
          ./users.nix
          ./packages.nix
          ./mail.nix
        ];
      };
    };
}