Mail: 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:

/bin/mail - from package: bsd-mailx
/usr/bin/Mail - from package: bsd-mailx
/bin/mail - from package: mailx
/usr/bin/Mail - from package: mailx

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

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