} // optionalAttrs (cfg.smtpHost != null) {
      EMAIL_SENDER_TRANSPORT = "SMTP";
      EMAIL_SENDER_TRANSPORT_host = cfg.smtpHost;

        '';
      };
      smtpHost = mkOption {
        type = types.nullOr types.str;
        default = null;
        example = ["localhost"];
        description = ''
          Hostname of the SMTP server to use to send email.