ucspi-proxy-pop3: 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/ucspi-proxy-pop3 - from package: ucspi-proxy

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

Error in other languages:
ucspi-proxy-pop3: bevel nie gevind nie
ucspi-proxy-pop3: comandă negăsită
ucspi-proxy-pop3: comando não encontrado
ucspi-proxy-pop3: commande introuvable
ucspi-proxy-pop3: command not found
ucspi-proxy-pop3: käsku ei ole
ucspi-proxy-pop3: không tìm thấy lệnh
ucspi-proxy-pop3: komanda nerasta
ucspi-proxy-pop3: Kommando nicht gefunden.
ucspi-proxy-pop3: kommandot finns inte
ucspi-proxy-pop3: komut yok
ucspi-proxy-pop3: nie znaleziono polecenia
ucspi-proxy-pop3: níor aimsíodh an t-ordú
ucspi-proxy-pop3: no se encontró la orden
ucspi-proxy-pop3: no s'ha trobat l'ordre
ucspi-proxy-pop3: opdracht niet gevonden
ucspi-proxy-pop3: parancs nem található
ucspi-proxy-pop3: perintah tidak ditemukan
ucspi-proxy-pop3: príkaz nenájdený
ucspi-proxy-pop3: příkaz nenalezen
ucspi-proxy-pop3: команда не найдена
ucspi-proxy-pop3: командата не е открита
ucspi-proxy-pop3: 命令找不到
ucspi-proxy-pop3: コマンドが見つかりません