pop3p: 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 CentOS possible default full paths are:

/usr/bin/pop3p - from package: 3proxy

You may install the required package with command yum install {package}

Error in other languages:
pop3p: bevel nie gevind nie
pop3p: comandă negăsită
pop3p: comando não encontrado
pop3p: commande introuvable
pop3p: command not found
pop3p: käsku ei ole
pop3p: không tìm thấy lệnh
pop3p: komanda nerasta
pop3p: Kommando nicht gefunden.
pop3p: kommandot finns inte
pop3p: komut yok
pop3p: nie znaleziono polecenia
pop3p: níor aimsíodh an t-ordú
pop3p: no se encontró la orden
pop3p: no s'ha trobat l'ordre
pop3p: opdracht niet gevonden
pop3p: parancs nem található
pop3p: perintah tidak ditemukan
pop3p: príkaz nenájdený
pop3p: příkaz nenalezen
pop3p: команда не найдена
pop3p: командата не е открита
pop3p: 命令找不到
pop3p: コマンドが見つかりません