mailer: 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
/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 Debian possible default full paths are:

/usr/sbin/mailer - from package: john

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