When you receive new email messages inside your inbox in Outlook Express, they will stay there until you either file them to another email folder, or delete them; as a security and protection measure of sorts, any email you trash will not actually be completely removed from Outlook Express: instead, it will go to the "Deleted Items" folder, which you may know from other email programs or webmail services as the "Trash" or "Recycle Bin" as it is called in Windows.
To keep the size of your email files small, you should regularly empty the Deleted Items folder: just right-click on it, and choose "Empty 'Deleted Items' Folder" from the context menu - at that point, Outlook Express will permanently delete these emails. But there is a setting, disabled by default, which allows you to make Outlook Express automatically empty the Deleted Items folder when it exits (when you click on the Close button, for example). This tutorial shows you how to turn on this option (which can be turned back off).
Tip: by default, Outlook Express will keep copies of messages inside the "Sent Items" folder; after some time, this folder may become larger than any other. Get into the habit of regularly deleting its content, and empty the Deleted Items afterwards (where deleted sent emails went).
Follow these steps to make Outlook Express take care of its own maintenance:
dbx" file: each of them can be compressed to use less space on your computer's hard drive: clicking on the button to open the "Local File Clean Up" dialog will let show how much percentage of space is wasted, and how much you'll save by compacting your folders.
Tip: if you right-click on the Deleted Items folder and the "Empty 'Deleted Items' Folder" menu item is disabled (grayed out), it simply means that there are currently no emails inside the folder!
Know that you can setup email rules (mail filters) to automatically delete messages based on many conditions: the "From" sender, a few keywords in the subject line, etc. A better alternative is to create a new folder called something like "Emails to Delete", and have Outlook Express move them in that folder: this gives you chance to review them before they're permanently gone! (Just delete them once you're convinced that this is their right fate, and optionally empty the trash when you are done.)