File extension
.mbox
mbox mailbox format
.mbox files are mailbox files: plain-text archives that store many e-mail messages in one file, originating on Unix systems and used by many mail clients.
Known meanings
Formats and reference material for .mbox
mbox mailbox format
Concatenated e-mail messages in one text file, each introduced by a From line
Details
Format notes
MBOX
The mbox format stores a collection of e-mail messages as one text file. It
comes from early Unix mail systems and has several incompatible variants. RFC
4155 documents the common form and registers the media type application/mbox.
The extension .mbox is used by clients such as Mozilla Thunderbird and for
exports from services such as Google Takeout, while Unix mail folders often
have no extension at all.
Structure
Messages are stored one after another. Each message begins with a separator line
that starts with the five characters From (with a space) followed by the
sender address and a date. The following lines are the message’s RFC 5322 header
and body. A blank line precedes the next separator. To keep body lines from being
mistaken for separators, lines in the body that start with From are escaped,
usually by prefixing >. The variants differ in how they do this: mboxo only
escapes From lines, mboxrd also escapes lines that already start with
>From so the process can be reversed, and mboxcl and mboxcl2 use a
Content-Length header instead.
Adoption
Many mail clients and mail servers can import and export mbox. Because the whole mailbox is one file, it is easy to copy and process with text tools, but it needs file locking when several programs access it and can be slow to modify or search when it becomes very large.
Preservation And Security Notes
Because of the variant differences, record which variant was used for an export and check that message counts match after migration. Mailboxes contain personal data and attachments encoded as MIME, so treat them as sensitive and scan attachments with the same care as any e-mail.
Further Reading
- RFC 4155, The application/mbox Media Type:
https://www.rfc-editor.org/rfc/rfc4155