[url=http://insideoe.tomsterdam.com/files/store.htm#storemain]Inside Outlook Express[/url] said:
Folders.dbx
This is the master index of an OE store folder and is required for OE to run. It stores the tree structure (nesting) of mail folders, the lists of newsgroups on each news account, and the synchronization options for all subscribed newsgroups and Hotmail folders. If it is missing when OE starts, OE will scan the store folder for all *.dbx files and then create a new Folders.dbx.
If Folders.dbx becomes corrupt, OE often stops displaying folders for existing *.dbx files, and sometimes will simply refuse to start. Also a bloated Folders.dbx can make OE run very slowly. Both problems can be repaired by closing OE and deleting Folders.dbx. Understand though that you will lose the mail tree structure, all lists of newsgroups, and all synchronization options, and that message rules will likely need to be edited.
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Inbox.dbx
Drafts.dbx
Sent Items.dbx
Deleted Items.dbx
These are the default mail folders, created automatically if missing. Should one of these stop working, move all messages into another folder, close OE, and delete the corresponding *.dbx file.
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Offline.dbx
This stores IMAP and Hotmail actions that you carry out offline, and are created only if an IMAP or Hotmail account exists
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Pop3uidl.dbx
This keeps track of messages left on a POP3 mail server, and is created automatically if missing. If OE starts downloading the same mail that you've already read, close OE and delete this file.
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Cleanup.log
This is created when automatic maintenance is enabled.
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<name>.dbx
This represents a user-created mail folder and its messages. The number of folders is limited only by your disk space, but don't let any one folder grow above 2Mb or so or performance is likely to suffer.
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<newsgroup>.dbx
This represents a newsgroup folder. All newsgroup folders are tied to the news account keys in the registry and so cannot normally be imported