How to integrate nixos-simple-mailserver into den?

[xdbr]

Sorry if this is too much of a noob question but maybe someone can answer it in a couple of keystrokes...

I'm struggling to include simple-nixos-mailserver into my dendritic setup.

If anyone can point me into the correct den-based-pattern to this usual flake-based pattern (see below)?

I've been wrapping stuff this way and that way, but without success: from all I can see, the mailserver attribute seems to not be evaluated?

Hint: one could also try setting mailserver.debug = true; as well as mailserver.stateVersion = null;. At least the latter should have a failing assertion when looking into the sources

{
  description = "NixOS configuration";

  inputs.simple-nixos-mailserver.url = "gitlab:simple-nixos-mailserver/nixos-mailserver/nixos-20.09"; # maybe better use tag 25.05 instead

  outputs = { self, nixpkgs, simple-nixos-mailserver }: {
    nixosConfigurations = {
      hostname = nixpkgs.lib.nixosSystem {
        system = "x86_64-linux";
        modules = [
          simple-nixos-mailserver.nixosModule
          {
            mailserver = {
              enable = true;
              # ...
            };
          }
        ];
      };
    };
  };
}

I've been banging my head for a long time now. Did it rot? Or am I overlooking something way too obvious?

[xdbr]

Finally, and after another round of woes and issues, I cracked it!

Basically, I got bitten by a missing hosts,... declaration in den.aspects.mailserver = {host, ...}: {, leaving the mailserver = { silently not being evaluated... Well, I learned it the hard way. But anyway, I'll leave it here as someone else's future reference. Nice thing is, it also uses sops-nix

{inputs, ...}: {
  den.aspects.mailserver = {host, ...}: {
    nixos = {config, ...}: {
      imports = [inputs.simple-nixos-mailserver.nixosModule];
      mailserver = {
        enable = true;
        stateVersion = 3;
        # debug = true;

        fqdn = "mail.example.com";
        domains = ["example.com"];

        loginAccounts = {
          "user@example.com" = {
            hashedPasswordFile = config.sops.secrets."mail.example.com/user/passwordHashed".path; # mkpasswd -sm bcrypt
            aliases = ["postmaster@example.com"];
          };
          # "user2@example.com" = { ... };
        };

        certificateScheme = "acme-nginx";
      };

      sops.secrets."mail.example.com/user/passwordHashed" = {};
    };
  };

  flake-file.inputs = {
    simple-nixos-mailserver = {
      url = "gitlab:simple-nixos-mailserver/nixos-mailserver/master"; # nixos-25.05
    };
  };
}