anchor.diff:  
     aliasesFile = pkgs.writeText "postfix-aliases" aliases;
     virtualFile = pkgs.writeText "postfix-virtual" cfg.virtual;
  +  checkClientAccessFile = pkgs.writeText "postfix-check-client-access" cfg.dnsBlacklistOverrides;
     mainCfFile = pkgs.writeText "postfix-main.cf" mainCf;
     masterCfFile = pkgs.writeText "postfix-master.cf" masterCf;
     transportFile = pkgs.writeText "postfix-transport" cfg.transport;
  
positive.code:     (mkIf haveVirtual {
        services.postfix.mapFiles."virtual" = virtualFile;
      })
    ]);
  
  }
  
negative.code:         description = "
            Entries for the transport map, cf. man-page transport(8).
          ";
        };
  
        dnsBlacklists = mkOption {
          default = [];
          type = with types; listOf string;
          description = "dns blacklist servers to use with smtpd_client_restrictions";
        };
  
        dnsBlacklistOverrides = mkOption {
          default = "";
          description = "contents of check_client_access for overriding dnsBlacklists";
        };
  
        extraMasterConf = mkOption {
  