There are two ways to archive: You can let Outlook automatically archive messages at regular intervals, or you can archive at times that you specify.
AutoArchive allows you to set up a schedule and archive multiple folders at the same time at set intervals. You'll know you have this feature turned on if periodically you get a friendly dialog box like the one in Figure 1. By default, Outlook turns on the AutoArchive feature.
The catch with AutoArchive is that it doesn't simply archive stuff, it can also delete it. If you want to keep everything, you should adjust your AutoArchive settings accordingly, as shown in Figure 2.
To change the AutoArchive settings, click Options on the Tools menu, click the Other tab, and click AutoArchive.