If you're running low on space, you can of course upgrade - otherwise, a little cleanup may be in order.
Are you saving mail on the server? If you are forwarding mail, be sure that it's not set to both forward and save a copy on the server. Be sure you've dumped your trash, too, if the webmail isn't set to do it automatically.
You can see where space is being used by logging into Cpanel then
From there you can clean up email