newmsgs: 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/share/doc/mh-book/examples/mh/bin/newmsgs - from package: mh-book

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

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