Sign in and go to "Mail", then click the "options" tab (it's a small one in the upper right-hand corner); it has a link for "junk mail protection" or something along those lines that lets you set your preferences for how much junk mail to block by adjusting the junk mail filter. However, I think there are only 3 options (low/enhanced/exclusive or something like that), so at least some spam is always being screened out even at the "low" setting; but if you check your junk mail file regularly you can avoid losing anything important (just make sure that in "delete junk mail" under the "options" tab, you have it set to to save the mail to that folder for a few days instead of automatically deleting junk mail).
