If your mailbox is full, there are a few things you can do to clear out the box and or adjust the size of your mailbox.
1. Login to webmail to view and delete your messages at: http://www.yourdomain.com.au/webmail (where yourdomain.com.au is your actual domain).
2. Login to the Snapfrozen client area, click on the cPanel button next your domain, then click on 'Email Accounts' to view and edit the settings for your email box.