What is a DBX File And Why Outlook Express Left Them Behind

A DBX store is one file per mail folder plus a master index that names them all. Both halves of that arrangement cause trouble. The index is a single point of failure for the whole store. Each individual file stops working at two gigabytes in a way that blocks the obvious fix.

Nobody chooses to deal with one. A DBX turns up on a machine somebody inherited. Sometimes in a folder named backup, on a drive that has been in a cupboard since the last decade. The mail inside is usually the only copy anywhere, which raises the stakes on every decision that follows. Where the file came from is worth establishing first. The wider set of mail formats shows how differently the alternatives behave.

One File Per Folder Plus an Index

Outlook Express gave every mail folder its own file, named after the folder. That much is easy to read from a directory listing, which is unusual for a binary mail format.

Mail is not all it kept there. Outlook Express doubled as a Usenet newsreader, so subscribed newsgroups and downloaded news articles sit in the same store under their own DBX files. A folder that looks like it holds a mailbox may hold years of newsgroup traffic beside it.

Folders.dbx          the index
Inbox.dbx
Outbox.dbx
Sent Items.dbx
Deleted Items.dbx
Clients.dbx          a folder somebody made
Offline.dbx          IMAP and Hotmail folders held locally
Pop3uidl.dbx         which messages were already downloaded

The naming makes the store look simple. It is simple, right up to the point where Folders.dbx is the file that goes missing.

Folders.dbx is not one folder among many. It records which files exist, what each one is called and how the folders sit inside each other. Every other file in the store is a folder. That one is the map.

Why Losing the Index Is Worse Than Losing a Folder

Lose Inbox.dbx and you have lost an inbox. Lose Folders.dbx and Outlook Express no longer knows that any of the others exist.

Anything built to parse the format can work around a missing index by reading each file directly. Outlook Express cannot, because it starts from the index and never looks anywhere else.

The Two Gigabyte Ceiling And Why It Traps People

Each DBX file stops at two gigabytes. That limit on its own would be an inconvenience. What makes it a trap is what happens when you reach it.

A full file refuses new messages, which is expected. It also refuses deletions, which is not. Outlook Express handles a deletion by marking the message and writing that change into the file. A file with no room cannot accept the write. So the action everybody takes first is the one action the file will not allow.

delete some old mail   blocked
empty deleted items    blocked
compact the folder     fails or corrupts
delete the file        works, mail gone

copy the whole folder  safe
move mail to a new
  folder in the app    often works
read the file directly safe
convert out           safe

The rename is the way out. Close Outlook Express, rename the oversized file to something else and start the program again. It finds the folder missing and creates a fresh empty one, which accepts writes because it has room. The renamed file still holds every message. File then Import brings it back as a separate folder rather than replacing what is there.

Error 0x800c0133 is a file problem wearing a network costume. It appears while sending or receiving, so it reads like a connection failure. What it usually means is that a DBX file is damaged or has reached its ceiling. Checking folder sizes answers it faster than checking the connection. The older crashes naming msimn or Directdb.dll point the same way.

Why the File Never Shrinks on Its Own

Deleting mail from a healthy DBX marks the space reusable inside the file without returning it to the disk. The file stays exactly as large as it was.

Outlook Express reclaims that space by compacting, which rewrites the store. On a file that has been left alone for years the recovery can be substantial. On a file near its ceiling, compacting is also where a good many stores were lost, since a rewrite of a fragile file is the worst moment for something to go wrong.

Copy before compacting, without exception. Compacting rewrites the file in place. A file old enough to be worth reading now is old enough to fail during that rewrite. What comes out the other side of a failed compact is less than what went in.

Where the Files Are Hiding

The store lives under application data in a folder named after an identifier rather than after anything readable. Nobody finds it by browsing.

Two reliable routes exist. While Outlook Express still runs, Tools then Options then the Maintenance tab has a Store Folder button showing the path directly. Where the program is gone and only the disk survives, the registry recorded the same path and still holds it.

Registry
  HKEY_CURRENT_USER
    Identities
      {GUID}
        Software\Microsoft\Outlook Express\5.0
          Store Root        <- the path

Typical result on Windows XP
  C:\Documents and Settings\[user]
    \Local Settings\Application Data
      \Identities\{GUID}
        \Microsoft\Outlook Express

Hunting by extension is the third route and the least pleasant. Searching a whole disk for files ending in dbx finds the store. It also finds every other program that happened to use those three letters. Confirm you have Folders.dbx sitting alongside before assuming you found mail.

One store is one identity, not one machine. Outlook Express let people keep separate identities on the same login, each with its own GUID folder and its own complete set of DBX files. Finding one store does not mean finding all the mail. The second identity is usually the one nobody remembers creating.

Why Outlook Express Left the Format Behind

Outlook Express shipped with Internet Explorer and reached most Windows users through Windows XP, where version 6 was the one almost everybody ran. Windows Vista replaced it with Windows Mail. Windows 7 dropped a built in mail client altogether.

Nothing that came afterwards reads a DBX. Windows Mail used a different arrangement. Windows Live Mail, which came after it, kept each message in an EML file of its own. Outlook has never read the format in any version.

Getting the Mail Out of One

1. Copy the entire store folder first. All of it, Folders.dbx included, before opening anything. Everything below assumes there is an untouched original to fall back to.
2. Check whether Folders.dbx is present. Its absence does not end the job, since the individual files can still be read, but it changes what any tool can tell you about folder structure.
3. Look at the file sizes. Anything close to two gigabytes explains most symptoms on its own. It also means the file should be read rather than repaired in place.
4. Pick the destination before converting. PST if the mail is going into Outlook, MBOX for Thunderbird, individual messages where each one has to stand alone.

Our file viewers read a store in a browser tab, which answers what is actually in there without installing anything or resurrecting a client from 2003. Where the messages need to end up somewhere a current program will open them, the mail converters cover those routes.