Glossary
What Is an MBOX File?
An MBOX file is an entire mailbox stored as one plain-text file. Each message sits inside complete — headers, body, and encoded attachments — stacked one after another, and a separator line beginning with From (the sender plus a timestamp) marks where every message starts. One .mbox file can hold ten emails or two hundred thousand.
Where MBOX files come from
- Google Takeout. Gmail's export format is MBOX — one big file for all your mail, or one per label. Our Gmail backup guide walks through the export.
- Apple Mail. Mailbox → Export Mailbox… writes an
.mboxpackage — the export path (and its gotchas) is covered in how to back up iCloud email. - Thunderbird. Every local folder is an MBOX file (no extension) inside the profile directory — see how to back up Thunderbird emails.
- Unix mail spools. The format started life in Unix in the late 1970s — which is why it's plain text, and why so much software can read it.
How to open an MBOX file
The classic route is importing it into a mail client — Thunderbird (with the ImportExportTools NG add-on) or Apple Mail's File → Import Mailboxes. If you just want to read or search it, skip the import: our free MBOX viewer opens even multi-gigabyte files in your browser without uploading anything. To break a mailbox into one portable file per message, use the MBOX to EML converter; to combine loose messages back into a mailbox, the EML to MBOX converter.
MBOX vs .eml
Same message format, different scope: MBOX is the box, .eml is one letter. Both store messages in the internet-standard RFC 822 format, but a .eml file holds exactly one message — which is why it's easier to open, attach, restore, and sync to cloud storage than a monolithic mailbox file.
| MBOX | .eml | |
|---|---|---|
| Holds | A whole mailbox | One message |
| Restore one email | Import or split it first | Open the file |
| Typical source | Bulk exports (Takeout, Apple Mail) | Single saves, per-message archives |
FAQ
How do I open an .mbox file without installing anything?
Drop it into InboxBucket's free MBOX viewer: it streams the file locally in your browser, builds a searchable index, and lets you read any message and extract it as a single .eml. Nothing is uploaded — even multi-gigabyte Takeout exports stay on your device.
Is MBOX the format Gmail exports?
Yes. Google Takeout delivers Gmail as MBOX — every selected message in one file. Note that Gmail itself can't re-import an MBOX; to get mail back into an account you'd sync it up through a desktop client over IMAP.
What's the difference between MBOX and PST?
Both pack a whole mailbox into one file, but MBOX is a plain-text open format that many clients and tools can read, while PST is Microsoft's proprietary binary container that effectively only Outlook opens.
Why do some lines in my MBOX file start with >From?
That's the format protecting itself. A body line that begins with "From " would look like the start of a new message, so exporters escape it as ">From". The MBOX variants (mboxo, mboxrd, mboxcl) differ mainly in the fine print of this escaping — viewers and importers handle it automatically.