re: I haven't used Express at all, but . . .
. . . just to add to alcaray's excellent advice, I haven't noticed that there's a limit to the 'outlook.pst' file (mine's got 76 mb as of this morning), which is where all the saved email goes, including attachments. I'm guessing Outlook Express creates something analogous, and will just keep appending to it as long as there's space on your hard drive(s).