courierpassd: command not found

Have You tried accessing the binary with the full path?

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/courierpassd - from package: courierpassd

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