Microsoft Outlook Question:

Outlook has a critical design flaw: If you don't archive or delete messages regularly, your file will grow too large for Outlook to handle. Eventually, Outlook will choke. If this happens, you can download a crop tool that will reduce the size of your Personal Folders file. Unfortunately, you'll lose data because this tool removes some to reduce the size of the file.

Avoiding the problem is the best defense:

Delete messages you don't need to keep.
Archive items regularly.