fdnsmxalist: 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/fdnsmxalist - from package: firedns

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

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