milter-greylist: 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 CentOS possible default full paths are:

/usr/sbin/milter-greylist - from package: milter-greylist

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/sbin/milter-greylist - from package: milter-greylist

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

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