Microsoft jumped on the spam filtering with Outlook 2003.  Unfortunately just like with many of their other products, they don't impliment it well and/or updates can change your settings.  You probably already see the "junk mail" folder.  First thing to do is check there for the emails you are missing.  However there may not be anything there.  To correct this we need to get into the junk mail settings.  To do that:

The descriptions are there for each setting.  Since most Outlook users are businesses we recommend using the "low" setting to keep from missing any mail that a client may send.  Our email servers actually block better than this setting but it's best if Outlook doesn't block things to strongly.

Underneath the level settings you will see 2 other options.  This first is "Permanently delete suspected E-mail..."  We highly recommend you keep this off.  If for some reason Outlook blocks a necessary email you will want to be able to retrieve it from the Junk Mail folder.

Once you have made those changes, click the OK buttons to get back to Outlook and start using your e-mail.