dkim-milter: 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/dkim-milter - from package: dkim-milter-python

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

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