pop3front-maildir: 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/sbin/pop3front-maildir - from package: mailfront

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

Error in other languages:
pop3front-maildir: bevel nie gevind nie
pop3front-maildir: comandă negăsită
pop3front-maildir: comando não encontrado
pop3front-maildir: commande introuvable
pop3front-maildir: command not found
pop3front-maildir: käsku ei ole
pop3front-maildir: không tìm thấy lệnh
pop3front-maildir: komanda nerasta
pop3front-maildir: Kommando nicht gefunden.
pop3front-maildir: kommandot finns inte
pop3front-maildir: komut yok
pop3front-maildir: nie znaleziono polecenia
pop3front-maildir: níor aimsíodh an t-ordú
pop3front-maildir: no se encontró la orden
pop3front-maildir: no s'ha trobat l'ordre
pop3front-maildir: opdracht niet gevonden
pop3front-maildir: parancs nem található
pop3front-maildir: perintah tidak ditemukan
pop3front-maildir: príkaz nenájdený
pop3front-maildir: příkaz nenalezen
pop3front-maildir: команда не найдена
pop3front-maildir: командата не е открита
pop3front-maildir: 命令找不到
pop3front-maildir: コマンドが見つかりません