postlog: 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/postlog - from package: postfix

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

On Debian possible default full paths are:

/usr/sbin/postlog - from package: postfix

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

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