rbldnsd: 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/rbldnsd - from package: rbldnsd

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

On Debian possible default full paths are:

/usr/sbin/rbldnsd - from package: rbldnsd

You may install the required package with command apt-get install {package}

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