mailnagd: 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/bin/mailnagd - from package: mailnag

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