{
  config,
  lib,
  pkgs,
  ...
}:
let

  cfg = config.services.mail-quota-warning;

in
{

  options = {
    services.mail-quota-warning = {

      enable = lib.mkOption {
        type = lib.types.bool;
        default = false;
        description = ''
          Enable mail-quota-warning daemon.
        '';
      };

      settings = lib.mkOption {
        type = lib.types.submodule {
          freeformType = with lib.types; attrsOf anything;
          options = {
            CHECK_INTERVAL_DAYS = lib.mkOption {
              default = 7;
              type = lib.types.int;
              description = ''
                Interval of days in which a warning message will be
                delivered.
              '';
            };
            QUOTA_WARNING_THRESHOLD_PERCENT = lib.mkOption {
              default = 80;
              type = lib.types.int;
              description = ''
                Threshold of used mailbox space in percent after which
                a warning message will be delivered.
              '';
            };
          };
        };
        default = { };
        description = ''
          Extra options which should be used by the mailbox quota warning script.
        '';
        example = lib.literalExpression ''
          {
            CHECK_INTERVAL_DAYS = 7;
            QUOTA_WARNING_THRESHOLD_PERCENT = 80;
          }
        '';
      };

      secretFile = lib.mkOption {
        type = with lib.types; listOf path;
        description = ''
          A list of files containing the various secrets. Should be in the
          format expected by systemd's `EnvironmentFile` directory.
        '';
        default = [ ];
      };

      interval = lib.mkOption {
        type = lib.types.str;
        default = "*:00,30:00";
        description = ''
          How often we run the sync. Default is half an hour.

          The format is described in
          {manpage}`systemd.time(7)`.
        '';
      };

    };
  };

  config = lib.mkIf cfg.enable {

    systemd.services."mail-quota-warning" = {
      description = "mail-quota-warning script";
      after = [ "network.target" ];
      wants = [ "network-online.target" ];
      environment = {
        PYTHONUNBUFFERED = "1";
      }
      // lib.mapAttrs (_: v: toString v) cfg.settings;
      serviceConfig = {
        Type = "simple";
        ExecStart = lib.getExe pkgs.mail-quota-warning;

        # hardening
        AmbientCapabilities = "";
        CapabilityBoundingSet = "";
        DevicePolicy = "closed";
        DynamicUser = true;
        LockPersonality = true;
        MemoryDenyWriteExecute = true;
        NoNewPrivileges = true;
        PrivateDevices = true;
        PrivateTmp = true;
        PrivateUsers = true;
        ProcSubset = "pid";
        ProtectClock = true;
        ProtectControlGroups = true;
        ProtectHome = true;
        ProtectHostname = true;
        ProtectKernelLogs = true;
        ProtectKernelModules = true;
        ProtectKernelTunables = true;
        ProtectProc = "invisible";
        ProtectSystem = "strict";
        RemoveIPC = true;
        RestrictAddressFamilies = [
          "AF_INET"
          "AF_INET6"
        ];
        RestrictNamespaces = true;
        RestrictRealtime = true;
        RestrictSUIDSGID = true;
        SystemCallArchitectures = "native";
        SystemCallFilter = [
          "@system-service"
          "~@privileged"
        ];
        UMask = "0077";
      }
      // lib.optionalAttrs (cfg.secretFile != [ ]) {
        EnvironmentFile = cfg.secretFile;
      };
    };

    systemd.timers.mail-quota-warning = {
      timerConfig = {
        OnCalendar = [
          ""
          cfg.interval
        ];
      };
      wantedBy = [ "timers.target" ];
    };

  };

  meta = {
    maintainers = with lib.maintainers; [ onny ];
  };

}