mboxgrep: 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/mboxgrep - from package: mboxgrep

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

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