anchor.diff:      (mkIf haveVirtual {
         services.postfix.mapFiles."virtual" = virtualFile;
       })
  +    (mkIf (cfg.dnsBlacklists != []) {
  +      services.postfix.mapFiles."client_access" = checkClientAccessFile;
  +    })
     ]);
   
   }
  
positive.code:     ''
      + optionalString haveVirtual ''
        virtual_alias_maps = hash:/etc/postfix/virtual
      ''
      + cfg.extraConfig;
  
  
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 {
  