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:     ''
      + optionalString haveVirtual ''
        virtual_alias_maps = hash:/etc/postfix/virtual
      ''
      + cfg.extraConfig;
  
  
negative.code:   haveAliases = cfg.postmasterAlias != "" || cfg.rootAlias != "" || cfg.extraAliases != "";
    haveTransport = cfg.transport != "";
    haveVirtual = cfg.virtual != "";
  
    clientAccess =
      if (cfg.dnsBlacklistOverrides != "")
      then [ "check_client_access hash:/etc/postfix/client_access" ]
      else [];
  
    dnsBl =
      if (cfg.dnsBlacklists != [])
      then [ (concatStringsSep ", " (map (s: "reject_rbl_client " + s) cfg.dnsBlacklists)) ]
      else [];
  
    clientRestrictions = concatStringsSep ", " (clientAccess ++ dnsBl);
  
    mainCf =
      ''
  