rmail.exim: 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.exim - from package: exim

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

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