mailfilter: 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/mailfilter - from package: mailfilter

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

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