ldns-mx: 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/bin/ldns-mx - from package: ldns

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

On Debian possible default full paths are:

/usr/bin/ldns-mx - from package: ldnsutils

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

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