postqueue: 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/postqueue - from package: postfix

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

On Debian possible default full paths are:

/usr/sbin/postqueue - from package: postfix

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

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