maildir-filter: 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/maildir-filter - from package: maildir-filter

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

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