Along similar lines to the Outlook thread Reading archived mails, I stumbled upon OE Viewer 3.0.0. This looks as if it would be handy to include on archive/ backup media, so that backed up .DBX files can be read using any machine, whether or not OE is installed:

"Viewer for standalone files containing Microsoft Outlook Express 4,5 and 6 message database (*.idx/*.mbx/*.dbx) and standalone EML files. This application is based on MiTeC Outlook Express Reader. It displays list of contained messages and their header. Message can be viewed in detailed view including attachments (save ability) and HTML preview and printing. Messages can be saved to *.eml files."