rmail: 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 - from package: exim

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

On Debian possible default full paths are:

/usr/bin/rmail - from package: postfix
/usr/sbin/rmail - from package: postfix
/usr/sbin/rmail - from package: courier-mta
/usr/sbin/rmail - from package: exim4-daemon-heavy
/usr/sbin/rmail - from package: exim4-daemon-light
/usr/sbin/rmail - from package: masqmail
/usr/sbin/rmail - from package: rmail

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

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