Since you are evidently receiving a large volume of emails you shouldn't really be using Outlook Express as it wasn't designed to cope with high volumes of email usually associated with an office environment. You should use Microsoft Outlook (part of Microsoft Office) or something similar that's designed to handle large volumes of email.
If you want to stick with Outlook Express you need to stop saving so many messages. Go through them regularly and delete those you don't really need anymore once they have served their purpose.
Just for your info, I use the program named Universal Extractor to unpack DBX files. I would suggest using the Binary Archive, because the installer version associates the program with DAT, BIN, DBX, and a couple of other file types.
You should be working with a COPY OF the DBX file in some folder other than your Outlook Express "store folder". I am sure you are.
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