To check your email usage on SI's mail server you can use SquirrelMail, but you may need to change some options to see your folder sizes. Once you've logged in to SquirrelMail click on the Options link:

Next, select 'Folder Preferences':

Scroll to the bottom of the right pane and check all of the bottom three check boxes, then click the 'Submit' button:

In the left pane click 'Refresh folder list':

Once the folder list has refreshed, scroll toward the bottom and look for and click the 'Folder Sizes' button:

The right pane will load and display your folders and the amount of space each folder takes up. You can scroll to the bottom of this list to find the total amount of disk space your email account is using: