pop_auth: 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/pop_auth - from package: solid-pop3d

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

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