rmail.sendmail: 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.sendmail - from package: sendmail

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

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