mailpost: 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/lib/news/bin/mailpost - from package: inn2

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

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