Emails stay on server after downloading

Emails stay on server after downloading

Hi, I'm wondering why my emails stay on the server when I download them using POP?? I use Thunderbird with the settings NOT to leave messages on the server. When I download my Zoho mail and later delete all it will be downloaded again after a while. It's a major problem....Also if I set Thunderbird to delete messages from server when I delete them locally they still stay and are re-downloaded after a while.

my address  squarebox@zoho.com