sensible-mda: 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/sbin/sensible-mda - from package: sensible-mda

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

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