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 feature but is limited to files up to 500 MB. Pro removes the size limit with a one-time in-app purchase. There is no feature gating — search, export, labels and threading all work in both.
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.