What is an .mbox file? The email archive format, explained
5 min read · Updated 2026-05-30
If you've exported email from Gmail, Apple Mail, or Thunderbird, you've probably ended up with a file ending in .mbox and wondered what it actually is. The short version: an .mbox file is a single plain-text container holding all the messages from one mailbox, in a format email programs have shared for decades.
The name is short for “mailbox.” Instead of storing each email as its own file, the mbox format concatenates every message into one long text file, one after another. A special line beginning with “From ” (with a trailing space) marks where each new message starts. Everything a message contains — the sender, recipient, subject, date, the body text, and any attachments — is written out as text inside that single file.
What's actually inside an .mbox file
Open an .mbox in a text editor and you'll see each message as a block of headers (From, To, Subject, Date and many technical ones) followed by the body. Attachments and non-English characters are encoded as text using schemes like Base64 and quoted-printable, which is why a photo shows up as a wall of letters and numbers rather than an image. It looks dense, but it's just text — there's no database, no proprietary container, nothing that needs special software to read in principle.
Why the format is everywhere
Because mbox is old, simple, and open, almost every serious mail program can read or write it: Thunderbird stores mail in it natively, Apple Mail exports to it, and Google Takeout hands you your Gmail as one. That universality is exactly why it's a good archive format — a .mbox you export today will still open in a dozen different programs years from now, unlike formats tied to a single vendor's app.
How to open and read one
An .mbox isn't meant to be read line by line in a text editor — it's meant to be loaded by a program that understands the format. A few options:
- Import it into a mail client. Thunderbird (with the ImportExportTools NG add-on) can import an .mbox and show the messages as a normal folder you can read and search.
- Scan it for what you need. If you only want to find something specific — like every subscription receipt — a tool that parses the format does the reading for you. sub-hunt.com reads the .mbox in your browser and surfaces the recurring charges, no import required.
- Peek at it as text. For curiosity, any text editor will open a small .mbox so you can see the structure — but large archives (gigabytes of mail) will overwhelm most editors.
Frequently asked questions
Is an .mbox file safe to open?
The file itself is plain text and can't run code, so opening it is low-risk. The usual caution applies to attachments inside it — but a tool that only reads text (like sub-hunt) never executes anything from the file.Why is my .mbox a folder on a Mac?
Apple Mail wraps the archive in a folder named “…mbox” that also contains index files; the real mbox archive is the file named “mbox” inside it. On other systems an .mbox is usually a single file.How big can an .mbox file get?
As big as the mailbox it holds — years of email with attachments can run to several gigabytes. Large exports are often split into multiple files for that reason.Can I get subscriptions out of an .mbox without reading it all?
Yes. That's exactly what sub-hunt.com does: it parses the .mbox in your browser and lists the subscriptions, amounts, and billing cadence, with a link to each source email — so you never have to scroll through the raw file.
Ready to scan your inbox?
Once you have your .mbox file, drop it into sub-hunt.com. Everything is parsed in your browser — nothing is uploaded.
Open the scanner