pop3test: 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/pop3test - from package: cyrus-imapd-utils

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

On Debian possible default full paths are:

/usr/bin/pop3test - from package: cyrus-clients

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

Error in other languages:
pop3test: bevel nie gevind nie
pop3test: comandă negăsită
pop3test: comando não encontrado
pop3test: commande introuvable
pop3test: command not found
pop3test: käsku ei ole
pop3test: không tìm thấy lệnh
pop3test: komanda nerasta
pop3test: Kommando nicht gefunden.
pop3test: kommandot finns inte
pop3test: komut yok
pop3test: nie znaleziono polecenia
pop3test: níor aimsíodh an t-ordú
pop3test: no se encontró la orden
pop3test: no s'ha trobat l'ordre
pop3test: opdracht niet gevonden
pop3test: parancs nem található
pop3test: perintah tidak ditemukan
pop3test: príkaz nenájdený
pop3test: příkaz nenalezen
pop3test: команда не найдена
pop3test: командата не е открита
pop3test: 命令找不到
pop3test: コマンドが見つかりません