ktnef: 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/ktnef - from package: kmail

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

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