ipop3d: command not found

Have You tried accessing the binary with the full path?

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/sbin/ipop3d - from package: uw-imap

You may install the required package with command yum install {package}
Error in other languages:
ipop3d: bevel nie gevind nie
ipop3d: comandă negăsită
ipop3d: comando não encontrado
ipop3d: commande introuvable
ipop3d: command not found
ipop3d: käsku ei ole
ipop3d: không tìm thấy lệnh
ipop3d: komanda nerasta
ipop3d: Kommando nicht gefunden.
ipop3d: kommandot finns inte
ipop3d: komut yok
ipop3d: nie znaleziono polecenia
ipop3d: níor aimsíodh an t-ordú
ipop3d: no se encontró la orden
ipop3d: no s'ha trobat l'ordre
ipop3d: opdracht niet gevonden
ipop3d: parancs nem található
ipop3d: perintah tidak ditemukan
ipop3d: príkaz nenájdený
ipop3d: příkaz nenalezen
ipop3d: команда не найдена
ipop3d: командата не е открита
ipop3d: 命令找不到
ipop3d: コマンドが見つかりません