smtp-source: 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/smtp-source - from package: postfix

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

On Debian possible default full paths are:

/usr/sbin/smtp-source - from package: postfix

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

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