If you use Outlook Express (or similar program) to download your emails to your computer, then view and delete them from there, you may occasionally get a "mailbox full" warning. This is because the emails will remain on the main server where they come from even though you have deleted them from your own computer.
This is convenient if you accidentally delete an email and then need to get it back because it will still be on the main server.
Emptying your mailbox is very simple. Go to the main server over the web by going to www.yoursite.com.au/webmail and log in with your details. Your username will be your full email address (you@yoursite.com.au) and the password will be the password of that particular email address.
After you have logged in you may delete the old emails that you no longer need.