mboxcheck: 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 Debian possible default full paths are:

/usr/bin/mboxcheck - from package: mboxcheck

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

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