greylistd: 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/sbin/greylistd - from package: greylistd

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

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