Automatic deletion of emails from server

I use the Plusnet webmail server and find that every so often have to manually empty the Plusnet inbox. I do not want to automatically deleting them from my computer. I have set up my Thunderbird account using pop3 and have tried all the combinations of settings in the heading...server settings..and checking and unchecking...... Leave messages on server For at most Until I delete them

and can not get the account to delete any emails from the server. Do I need to set something else under disk space? When you set For at most 14 days, is that 14 days from the email arriving at the computer, or 14 days from setting it to 14 days? Is there something that a am failing to do ? Any help gratefully received.