getmail_mbox: 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/bin/getmail_mbox - from package: getmail

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

On Debian possible default full paths are:

/usr/bin/getmail_mbox - from package: getmail4

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

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