Corrupted Outlook Express e-mail files

Understanding Outlook Express Files
Signs and Symptoms of DBX Corruption
When Special Folders Stop Being Special
The Show-Stopper: Folders.dbx
Viral Irony: The Most Common Cause of Corruption
When the Cure Kills: Compacting and Corruption
Minimizing the Risk Is Also Minimizing the Loss
Recovering from File Corruption
Ten Steps of Prevention

See:
http://www.microsoft.com/windows/IE/community/columns/filecorruption.mspx