Your email box is full. You will need to login to webmail to see  how full your email box is and delete email: using your email credentials to delete mail to clear out your email box. In order to purge the mail and free up space you will need go to settings and uncheck the 2 boxes under Trash setting. Then delete as much email as you can (The email will not be move to trash and will be deleted permanently). After you have freed up some space you will be able to receive email again. In order to enable the trash again you can check the trash options in settings again. There is a 1GB limit. To prevent this we recommend you setting your email client to not leave mail on the server for longer than ~7 days.