postmulti: 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/postmulti - from package: postfix

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

On Debian possible default full paths are:

/usr/sbin/postmulti - from package: postfix

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

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