fdnsmx: 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 Debian possible default full paths are:

/usr/bin/fdnsmx - from package: firedns

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

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