sendmail.ssmtp: 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 CentOS possible default full paths are:

/usr/sbin/sendmail.ssmtp - from package: ssmtp

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