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

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

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