mail-notification: 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/mail-notification - from package: mail-notification

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

On Debian possible default full paths are:

/usr/bin/mail-notification - from package: mail-notification

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

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