smtp: 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:

/sbin/smtp - from package: apcupsd

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

On Debian possible default full paths are:

/sbin/smtp - from package: apcupsd

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

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