This means that your mailbox is full. Each mailbox has a limited size. You should remove older emails on the server so you can start receiving mails again. There is no point in removing mails in your local POP3 mail client.
A full mailbox may have various reasons:
•You received very large emails and you have not yet removed them from the server.
•You leave your mail on the server. This happens when you only use webmail to read your mail, or when you access the server trough IMAP or if you access the server trough POP3 but you have checked the option "leave a copy of message on the server".
•Also, pay attention when you create extra folders for your large mails. Maybe your inbox is empty and yet your mailbox is full. These folders also take up space!
There is limited tolerance. If you exceed this limit, incoming mail will be blocked as long as you use too much space.