lockmail.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/lockmail.maildrop - from package: maildrop

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

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