ssmtp: 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/sbin/ssmtp - from package: ssmtp

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

On Debian possible default full paths are:

/usr/sbin/ssmtp - from package: ssmtp

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

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