rmail.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/bin/rmail.postfix - from package: postfix

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

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