mail: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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