miltertest: 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/miltertest - from package: opendkim-tools

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

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