maildirkw: 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/maildirkw - from package: courier-base

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

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