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Data format

MBOX converter

An .mbox file is a mailbox: many email messages concatenated into one plain-text file, each introduced by a line beginning with the word From followed by a space. It is what a mail export from Gmail or Thunderbird gives you.

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Drop MBOX files here

Batch files can each use a different output. Nothing uploads for local conversions.

Working inputs include camera RAW, browser-local audio/video, PDF, CBZ/CBR comics, office documents, ebooks, markup, 3D models, structured text, images, and archives.

Where MBOX comes from

The mbox format dates to Version 6 Unix in the 1970s, where a user's mail was simply a file appended to. It was never standardised — RFC 4155 documents it descriptively rather than normatively — and split into incompatible variants (mboxo, mboxrd, mboxcl, mboxcl2) over how to handle a body line that itself begins with From. Google Takeout still exports mbox, which is why the format remains a live concern rather than a historical one.

How MBOX works

  • Messages are separated by a From_ line — literally From, a space, an address and a timestamp — at the start of a line.
  • A body line beginning with From must be escaped, and the variants disagree on how, which is the format's central defect.
  • Each message after its separator is an ordinary RFC 5322 message, headers and MIME included.
  • The file has no index, so finding one message means scanning from the beginning.
  • Concurrent writers require file locking, historically a source of corruption.

When to use MBOX

  • Bulk email export and import between providers
  • Long-term archival of a mailbox as a single file
  • Migrating mail between clients or servers
  • Extracting a mailbox into structured data for search or review

Strengths and limitations

Strengths

  • One file holds an entire mailbox, simplifying backup and transfer
  • Plain text, so it survives any transport and can be inspected directly
  • The de facto export format of most large mail providers

Limitations

  • Incompatible variants disagree on From-line escaping, which can corrupt message bodies
  • No index, so random access requires a full scan
  • A single file for a large mailbox becomes unwieldy and risky to edit

Compatibility

Imported by Thunderbird, Apple Mail and most migration tools, and produced by Google Takeout. Novus Convert splits the mailbox locally and converts to JSON, or to a ZIP containing each message as its own file, which is usually the more practical shape for review.

Per-message headers and text bodies are preserved. The From_ separator lines are structural and are not carried into the output.

Convert MBOX to another format