Your Mailhub service needs to be set to "store". The Webmail e-mail client can only read your mail stored on the Mailhub systems. If you "forward" your e-mail to another mail server, you will need to use another e-mail client to read that e-mail. Additionally, if your Mailhub service is set to "reject", then you will not be able to use the Webmail e-mail client.
Click here for more information regarding your Mailhub service.