postmap: 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/postmap - from package: postfix

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

On Debian possible default full paths are:

/usr/sbin/postmap - from package: postfix

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

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