maildir2mbox: 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/maildir2mbox - from package: qmail
/var/lib/qmail/bin/maildir2mbox - from package: qmail

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

Error in other languages:
maildir2mbox: bevel nie gevind nie
maildir2mbox: comandă negăsită
maildir2mbox: comando não encontrado
maildir2mbox: commande introuvable
maildir2mbox: command not found
maildir2mbox: käsku ei ole
maildir2mbox: không tìm thấy lệnh
maildir2mbox: komanda nerasta
maildir2mbox: Kommando nicht gefunden.
maildir2mbox: kommandot finns inte
maildir2mbox: komut yok
maildir2mbox: nie znaleziono polecenia
maildir2mbox: níor aimsíodh an t-ordú
maildir2mbox: no se encontró la orden
maildir2mbox: no s'ha trobat l'ordre
maildir2mbox: opdracht niet gevonden
maildir2mbox: parancs nem található
maildir2mbox: perintah tidak ditemukan
maildir2mbox: príkaz nenájdený
maildir2mbox: příkaz nenalezen
maildir2mbox: команда не найдена
maildir2mbox: командата не е открита
maildir2mbox: 命令找不到
maildir2mbox: コマンドが見つかりません