It sounds as if your Inbox file is corrupted
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When Special Folders Stop Being Special
When a special folder becomes corrupt, the first sign is usually just that the folder stops working. For example, you might find that messages you send are no longer being saved in the Sent Items folder or that you are suddenly unable to delete messages, or you cannot open messages in the Drafts folder. If the Inbox is corrupt you will usually see just an empty folder, although sometimes new e-mail will not be downloaded. In the case of the Sent Items or Outbox folders, Outlook Express might crash when you attempt to send a message, presenting this error:•
Unknown error has occurred. Protocol: SMTP Port: 0 Secure: No [SSL] Error: 0x800c0131
If the Inbox is corrupt, Outlook Express might not start, but in most cases it will do so by creating a new DBX file named Inbox(1).dbx.
In all these cases compacting all folders often fixes the damage. If it fails to do so, you will need a new DBX file for the damaged folder.
If possible, drag all messages in the non-working folder into another folder you create just for this purpose. You can create a folder by pointing to Folder on the File menu, and then clicking New folder (or by pressing Ctrl+Shift- E).
Close Outlook Express.
Open your Store Folder in Windows Explorer.
Delete (or rename) the DBX file associated with the non-working folder. For example, "Sent Items.dbx" or "Deleted Items.dbx" or "Drafts.dbx". Since these are default folders, new DBX files will be created automatically for the special folders when you restart Outlook Express.