Synchronisation problems

When I receive mail, the original copy remains on the server. Ultimately the server mailbox becomes full and we cannot receive further emails. When I delete all of the emails on the server I lose all of the emails on my computer. I have unchecked the box in “settings” so that synchronisation does not occur, but this hasn’t solved the problem. Is there a solution to this?

You have to use POP3 protocol instead of IMAP.
Edit: If you want to configure your account manually, open Tools - Accounts - New account - Mail - Other and click Next.

Thank you very much for your help. It has fixed the problem.