postfix: 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/postfix - from package: postfix

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

On Debian possible default full paths are:

/usr/sbin/postfix - from package: postfix

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

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