An Email No No!
Important Note: If you are still using Outlook Express, I urge you to migrate to another safer email program. There are several email programs that are free and will not cause as many problems as Outlook Express. A popular replacement for Outlook Express is Thunderbird.
Some people have put their email folders in the Inbox! This is a recipe for disaster. Problems with email generally occur when email clients get too big. The first thing to get corrupted is usually the Inbox. When corruption occurs the fix is usually to delete the Inbox folder.
If your email program gets in trouble, especially Outlook Express, the fix is often to delete the Inbox and let the program create a new Inbox the next time it is opened. Guess what happens if you put your folders in the Inbox instead of the top level for folders. Yep, you probably guessed it, all of the folders in the Inbox get deleted as well.
So if you are one of the culprits that have put all of your folders in the Inbox, move them to the top folder level and I suggest you do it now. It is easy to move the folders. You can just drag them one by one to the top level.
The top level in Outlook Express is named 'Local Folders'. You can find Local Folders listed just below the Outlook Express icon. In my installation of Thunderbird there are two top levels. One is my Gmail account name and the other is named Local Folders.
There are a lot of other email clients like Courier, Incredimail, etc. I am not familiar with those programs so I can't say for sure how easily they become corrupted or what the 'fixes' would be if they need fixing. But some of these programs may be built on the back of Outlook Express (No I don't know which if any are built on OE) and so any problems would require the deletion of the Inbox for them as well.
The best thing is just don't take any chances and don't store too much in the Inbox and that includes folders!
Move those really important messages to an archive away from the possibility of losing them.
While you're at it, don't forget to make a backup of all your email.