rbldns: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/sbin/rbldns - from package: ndjbdns

You may install the required package with command yum install {package}

Error in other languages:
rbldns: bevel nie gevind nie
rbldns: comandă negăsită
rbldns: comando não encontrado
rbldns: commande introuvable
rbldns: command not found
rbldns: käsku ei ole
rbldns: không tìm thấy lệnh
rbldns: komanda nerasta
rbldns: Kommando nicht gefunden.
rbldns: kommandot finns inte
rbldns: komut yok
rbldns: nie znaleziono polecenia
rbldns: níor aimsíodh an t-ordú
rbldns: no se encontró la orden
rbldns: no s'ha trobat l'ordre
rbldns: opdracht niet gevonden
rbldns: parancs nem található
rbldns: perintah tidak ditemukan
rbldns: príkaz nenájdený
rbldns: příkaz nenalezen
rbldns: команда не найдена
rbldns: командата не е открита
rbldns: 命令找不到
rbldns: コマンドが見つかりません