dkim_responder: 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_responder - from package: dkimproxy

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

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