deliverquota.maildrop: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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/sbin/deliverquota.maildrop - from package: maildrop

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