fdnsmxlist: 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/fdnsmxlist - from package: firedns

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

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