What is the DBX file format?
.dbx files store the contents of a mail folder created by Microsoft Outlook Express, with each folder kept as a separate binary file along with B-tree index data for faster message access. .dbx files have a hard upper size limit of 2 GB, but the safest and most practical size is up to 300 MB - larger files can easily get corrupted, even during the compacting process. The format is proprietary, developed by Microsoft for Outlook Express, an email client no longer included in Windows systems since Windows 7.
Each .dbx file begins with the 4-byte signature CF AD 12 FE at offset 0. Deleted messages remain inside the file and continue to occupy space until the store is explicitly compacted. The .dbx format replaced the earlier Outlook Express 4 pair of .MBX and .idx files, and was itself frozen when Outlook Express 6 was discontinued.
Outlook Express by default creates a .dbx file for each mail folder per user. Default program settings configure the folder location to be Documents and Settings, where users can find the following files:
Folders.dbx- stores the index of all mailboxes and is used to restore the file structure in case of corruption or loss.Offline.dbx- access data for IMAP or Hotmail accounts.Inbox.dbx- inbox messages.Sent Items.dbx- sent messages.Drafts.dbx- message drafts.Deleted Items.dbx- the recycle bin for Outlook Express messages.
Messages stored in .dbx mailbox files can be imported into applications that replaced Outlook Express, namely Windows Mail and Windows Live Mail, using the appropriate menu option. Many third-party tools for converting .dbx into other formats - for example .PST or .EML - are available. Note that .dbx is entirely distinct from Outlook's .pst format; Outlook Express and Microsoft Outlook are different programs. .dbx files should not be opened manually, as doing so may cause Outlook Express not to function properly or result in file corruption. Recovering lost data requires use of specialized tools and it may not always be possible to recover everything.
Security & safety
RISK: MEDIUMA .dbx is passive data, but the mail inside is the real exposure: old e-mail can hold saved passwords, personal data and live attachments - treat an inherited store as sensitive. Old attachments may contain dormant malware, so scan before opening anything extracted. The format is fragile near its 2 GB limit, so always work on a COPY of the store and never the original. Avoid sketchy "free DBX viewer" download sites; stick to DBXtract (oehelp.com), Aid4Mail or DataNumen.
Format details
in a nutshell- Visual FoxPro / DataBase indeX - Some database tools use .dbx for a database or index file - unrelated to Outlook Express mail.
- DesignXpress / AVG / Cisco config files - Various apps have reused .dbx for project, antivirus or device-config data; the database here is the Microsoft Outlook Express mail store.
Programs that open DBX files
Windows4 apps
Windows Live Mail
Free Last client that imports Outlook Express stores: File > Import messages > Microsoft Outlook Express 6. (Discontinued by Microsoft - install only from trusted archives.) 
Aid4Mail
Paid Convert/migrate DBX stores into PST, MBOX, EML or directly into Outlook/other clients. 
DBXtract
Free Extracts individual messages from a .dbx (even damaged ones) to .eml files you can open anywhere - the classic recovery tool.
DataNumen Outlook Express Repair
Paid Repair and recover messages from corrupted .dbx files (e.g. ones that hit the 2 GB limit). Technical details
deep spec| File structure | Binary B-tree store; each mail folder is kept in its own `.dbx` file |
| Magic bytes | `CF AD 12 FE` at offset 0, followed by a type marker that distinguishes message stores, the folder index, and control files |
| Byte order | Little-endian |
| Hard size limit | 2 GB per `.dbx` file; corruption risk increases significantly as the file approaches this ceiling |
| Compression | None - message bodies are stored uncompressed |
| Encryption | None - content is stored in plain binary form on disk |
| MIME type | `application/octet-stream` (also recognized as `application/x-dbx`) |
| Message format stored | Raw RFC 822 / MIME message bodies, stored after the B-tree index region |
| Deleted message handling | Deleted messages remain physically inside the file and occupy space until the store is manually compacted |
| Master index file | `Folders.dbx` holds the index of all mail folders and is used to reconstruct the full folder tree |
| Format lineage | `.dbx` replaced the OE4 `.mbx` / `.idx` pair (introduced 1999) and was frozen at Outlook Express 6; incompatible with the earlier OE4 format |
| Associated platform | Windows 98, Windows 2000, Windows XP; Outlook Express was removed starting with Windows Vista |
| Superseded by | Windows Mail (stores messages as individual `.eml` files, Vista+) and Windows Live Mail; Microsoft Outlook uses `.pst` / `.ost` |
| Default storage path | User profile under `Documents and Settings\[username]\Local Settings\Application Data\Identities\` |
| Repair and recovery | Corruption is common as files approach the 2 GB limit; dedicated DBX repair tools are required since manual editing will corrupt the binary store |
| Released | 1997 (Outlook Express 4 / 5 message store; .dbx introduced with Outlook Express 5, 1999) |
| Latest version | Outlook Express 6 (2001); format frozen - Outlook Express discontinued after Windows XP |
| Specification | en.wikipedia.org |
DBX conversions
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