What is the MBX file format?
An .mbx file is an email mailbox storage file used by several older email clients, most notably Eudora (Qualcomm), Outlook Express (Microsoft), Pegasus Mail, and PocoMail. It belongs to the mbox family: messages are stored as concatenated plain text, each separated by a From (From-space) header line per the convention defined in RFC 4155. This makes .mbx files readable in a plain text editor, though large mailboxes can reach several gigabytes.
Each message follows the RFC 5322 format - headers then body - with MIME attachments encoded inline as Base64 or quoted-printable. Implementation details vary by client:
- Eudora: strict mbox format; closest to the RFC 4155 standard
- Outlook Express:
.mbxpaired with a companion .DBX index file; without the.dbx, the mailbox may still be readable as raw mbox text - Pegasus Mail:
.mbxpaired with a separate.pmmindex file - PocoMail: mbox-compatible variant
Eudora was discontinued in 2006. Outlook Express was last bundled with Windows XP and is absent from Windows Vista and later. Mozilla Thunderbird (with the ImportExportTools NG add-on) and dedicated migration tools such as MailStore Home can import .mbx files.
Security & safety
RISK: LOWPlain-text mailbox container. The emails inside may contain phishing links or malicious attachments - standard email safety rules apply when opening message content. The MBX file itself has no executable code.
Format details
in a nutshell- Netscape Mail / Mozilla Mail mailbox - Netscape Communicator and early Mozilla Mail used .mbx for mailbox storage; Netscape browser suite discontinued 2008.
Programs that open MBX files
Windows3 apps
Pegasus Mail
Free Use 'Add mailbox to list' feature, point to the directory containing MBX files. Pegasus Mail reads MBX folder stores directly. 
Mozilla Thunderbird + ImportExportTools NG
Open-source Install ImportExportTools NG add-on in Thunderbird; right-click Local Folders → Import mbox file → select .mbx file. Thunderbird reads mbox/mbx natively.
MailStore Home
Free Import from Eudora or Outlook Express via built-in migration wizard; handles MBX/TOC and MBX/DBX pairs. 
macOS1 app
Mozilla Thunderbird + ImportExportTools NG
Open-source Same as Windows - import .mbx file using ImportExportTools NG add-on. 
Linux1 app
Mozilla Thunderbird + ImportExportTools NG
Open-source Import .mbx as mbox file via ImportExportTools NG add-on. Technical details
deep spec| Format family | mbox (RFC 4155) |
| Message separator | From (From-space) line at the start of each message |
| Encoding | Plain text (ASCII / UTF-8 / ISO-8859-1 per message content-encoding) |
| Attachment encoding | Base64 or quoted-printable (inline MIME within the text stream) |
| Typical file size | 1 MB - 2 GB depending on mailbox age and message count |
| Outlook Express variant | Paired with a companion .dbx index file; messages may still be readable without it |
| Pegasus Mail variant | Paired with a separate .pmm index file |
| Associated OS | Windows, macOS (Eudora for Mac) |
| Eudora EOL | 2006 (Qualcomm discontinued; Aurora fork exists as alpha) |
| Outlook Express availability | Last shipped with Windows XP; absent from Windows Vista and later |
| Released | early 1990s (Eudora 1.x for Mac/Windows) |
MBX conversions
Community Q&A
asked by usersNo questions yet - be the first to ask about MBX files.
