Frequently asked questions
Everything about MBOX files, Thunderbird and Google Takeout exports, Apple Mail archives and how Mbox Viewer works.
What is an MBOX file?
MBOX is a standard format for storing email messages in a single file. Google Takeout exports your Gmail as MBOX, and many email clients and servers use it for backups and archives.
How do I back up my Gmail as MBOX?
Go to takeout.google.com, deselect everything, select only “Mail”, and create the export. Google prepares a .zip archive (it can take hours for large accounts). Download it, unzip it, and open the .mbox file with Mbox Viewer.
Can it open very large files?
Yes. Mbox Viewer uses a streaming parser that never loads the whole file into memory, so it opens archives of 50 GB and beyond. The first open builds an index; every later open is under a second.
Does it modify my MBOX file?
Never. Mbox Viewer is strictly read-only. It creates a small index file next to your MBOX for fast access, but never writes to the original.
What's the difference between Free and Pro?
Free includes every reading tool — search, Gmail labels, threading, the MIME and hex views — and is limited to mailboxes up to 1 GB. Pro lifts that limit and adds three things: batch export to PDF and HTML, repairing damaged mailboxes, and merges above 1 GB combined. Export to EML, CSV, text and MBOX, and pulling out attachments, work the same in both. Pro is a one-time in-app purchase.
Is there a subscription?
No. Pro is a single one-time purchase through the Mac App Store (or the Microsoft Store on Windows). No subscription, no account, no recurring charges.
Does it support HTML emails?
Yes. Mbox Viewer renders HTML emails with their original formatting. For privacy, remote images are blocked by default, with an option to load them, and message HTML is sanitized before display.
Which macOS versions are supported?
macOS 14 Sonoma and later. It's a Universal Binary that runs natively on Apple Silicon (M1–M4) and Intel Macs.
Is there a Windows version?
Yes. A native Windows app is available on the Microsoft Store, with the same features as the Mac app — including 18 languages and files up to 50 GB.
Does my data leave my Mac?
No. Everything runs locally — no cloud, no telemetry, no account. Your emails never leave your machine.
How do I open a Thunderbird mbox file on Mac?
Thunderbird stores each mailbox (Inbox, Sent…) as a standard mbox file without a file extension. Mbox Viewer opens these extensionless mailboxes directly — just point it at the file in your Thunderbird profile folder, no renaming to .mbox needed. Files exported with add-ons like ImportExportTools NG already carry the extension and open the same way.
How do I open an Apple Mail export (.mbox folder)?
Apple Mail exports a folder named “YourMailbox.mbox” that contains a file called simply “mbox”. Mbox Viewer opens the .mbox export package directly — just select (or drag & drop) the “YourMailbox.mbox” folder and it finds the messages inside automatically. No need to dig out the inner “mbox” file.
Can I view my Gmail / Google Takeout export?
Yes. Google Takeout delivers your Gmail as a standard .mbox file that Mbox Viewer opens directly — including your Gmail labels, which show up in the sidebar. Unzip the Takeout archive and open the .mbox file inside.
Can I open Outlook emails (PST / OLM)?
Not directly — Outlook stores mail in PST/OLM, which isn't mbox. Convert the file to mbox with a converter tool first, then open the result with Mbox Viewer.
Can a school or a company buy licences in volume?
Yes, through a separate edition: Mbox Viewer Pro for Education, a Custom App bought by the seat in Apple School Manager or Apple Business Manager and deployed by MDM, with everything unlocked and no in-app purchase. It is not on the public App Store: an organization is added by its Organization ID first.
I double-clicked my MBOX file and nothing happened. Why?
Because macOS and Windows ship with no app registered for the .mbox extension, so the double-click has nowhere to go — or it hands the file to Apple Mail, which tries to import it instead of opening it. Installing Mbox Viewer registers the extension: from then on a double-click opens the mailbox for reading, without importing it anywhere.
Can I save emails from an MBOX file as PDF?
Yes. Any message can be printed or saved as PDF with its headers intact, and Pro does it in batches for a whole selection. You can also export messages as EML, as plain text, as CSV, or as a brand-new MBOX file containing only what you picked.
Is it safe to open an MBOX file?
An MBOX file is plain text, so it cannot run anything by itself; the risk lives in what you do with what is inside — an attachment you save and open, or a remote image that tells a sender you read the message. Mbox Viewer is read-only and never writes to your archive, and it blocks remote images by default, so nothing loads from the internet unless you ask for it.
How big can an MBOX file get before it stops opening?
There is no ceiling in the format itself, and none in the app: a 50 GB archive of around half a million messages opens in under five minutes and re-opens in under a second, using less than 600 MB of RAM. What most other readers hit is their own limit, not the file's — they load the whole thing into memory before showing anything. The free version opens mailboxes up to 1 GB; Pro removes that limit.