maildrop: 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/maildrop - from package: courier-maildrop
/usr/bin/maildrop - from package: maildrop

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

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