filterctl: 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/filterctl - from package: courier-mta

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

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