If you opted for a POP configuration, all e-mails will remain in your online mailbox / server, even if you delete them in your e-mail program. After a while the server is so full that you have to delete the old e-mails.
That is why we recommend that you set up your e-mail program so that the e-mails that you delete are also deleted from the server. View our manuals.
Is your mailbox already full? Then first delete redundant emails via webmail