# Moving mbox files from Windows to Mac ## What does moving mbox files between systems actually involve? An mbox file is essentially one large text container that stores all of your emails one after another. Programs such as Thunderbird, macOS Mail and many archiving tools use this format. The good news is that mbox is an open, platform independent format: an mbox file you created on Windows is the exact same file on a Mac. There is no separate Windows version or Mac version of the format. In practice you still run into small differences between the two systems. Those differences are not in the mbox format itself, but in how Windows and macOS handle files: different line endings in text, different characters allowed in a file name, and programs that sometimes write a slightly different variant of the format. In this article we explain what to watch out for, and how to open and verify the archive locally on both systems with Mbox Viewer. ## Why Mbox Viewer helps here The usual route is to let your email program on the Mac import the mbox file. That works, but such an import can fail on a file that is too large, an unexpected character in the content, or a variant of the format the program does not fully understand. And you have to import everything before you even know whether the transfer went well. Mbox Viewer flips that order around. You simply drag the file into the window and read it right away, without importing and without it ending up in a mailbox. That lets you check on the Mac that the archive is complete and readable before you import it anywhere permanently. A nice detail: everything happens locally in your browser. Nothing is uploaded, there is no server in between and there is no telemetry. The extension's Chrome permissions are empty and your archive stays on your own device. :::info title="Mbox is the same on both systems" You do not need to convert the file. A .mbox or .mbx from Windows is still just an mbox on a Mac. The points below are about the transfer and about small differences in text formatting, not about the format itself. ::: ## What to watch out for between Windows and Mac Before you start, here are a few differences that can disrupt an import elsewhere. With Mbox Viewer you usually notice little of this, because it reads the content cleanly, but it is good to know them. - Line endings: Windows and macOS mark the end of a line differently. Some older import tools stumble over this. Mbox Viewer reads both variants without you having to do anything. - File name: do not use special characters (such as : or /) in the name. macOS and Windows do not allow exactly the same characters, and a name that is fine on one system may cause problems on the other. - Character sets (encoding): emails with accents, emoji or non-Latin characters can look strange after a rough conversion. So always move the file unchanged, meaning do not run it through a text editor that saves it again. - Size: an mbox archive can become very large. Keep this in mind when choosing your transfer method and when setting the memory limit in Mbox Viewer. :::warn title="Never open the mbox in a text editor to edit it" An mbox file looks like plain text in a text editor, but saving it again from such an editor can silently change the line endings or the character set. That can unintentionally make the emails unreadable or damage attachments. Always move the file as a whole, unchanged. ::: ## Moving the file from Windows to Mac :::howto title="Copy an mbox file from Windows to Mac" 1. Locate the mbox file on your Windows PC. In Thunderbird you find these in the profile folder under Mail; macOS Mail and archiving tools often produce a separate .mbox file directly. 2. Choose a transfer method: a USB stick that works on both systems, an external drive, or a cloud drive folder. If you want to stay strictly local, use a USB stick or cable rather than a cloud service. 3. Copy the file unchanged to the chosen medium. Do not zip and unzip it with dubious tools, and do not open it in an editor along the way. 4. Connect the medium to your Mac or open the shared folder, and copy the file to a fixed location, for example your Documents folder. 5. Check that the file size on the Mac matches the size on the Windows PC. If it differs significantly, the copy may be incomplete; copy it again. ::: :::tip title="Using Google Takeout?" If you got your email from Google Takeout, you often receive a compressed .mbox.gz file. You do not have to unpack it yourself first: just drag it into Mbox Viewer and it will be unpacked and read automatically. ::: ## Opening and verifying the archive on the Mac :::howto title="Open and verify an mbox in Mbox Viewer" 1. Open Mbox Viewer in Chrome on your Mac. You can find more information and the extension itself at https://mbox-viewer.online. 2. Drag the transferred mbox file from Finder into the extension's window. The messages appear in a list on the left, with the selected message on the right, similar to Gmail or Thunderbird. 3. Scroll through the list and check that the number of messages is correct and that the oldest and newest emails look right. Use the j and k keys to browse quickly. 4. Open a message with accents or attachments and look at the Preview tab. If the text looks right and the attachments are visible, the character set came across correctly. 5. If you have doubts about a specific message, check the Raw tab to see the unprocessed source exactly as it appears in the file. 6. If you want to keep or pass on the archive on the Mac in another format, export per message or in bulk to, for example, PDF, .eml or mbox again. ::: :::info title="External images stay blocked" When reading, external images are not loaded by default. That protects you from hidden tracking pixels that could register that you are opening an old email. If you do want to see the images, you load them per message with a button. ::: ## Large archives and memory Mbox archives holding years of email can be quite large. Because Mbox Viewer reads everything in your browser, there is an adjustable memory limit. If you hit that limit with a very large file, you can change the in-memory limit in Settings. A cleaner solution is often to split the archive into smaller mbox files and open them one by one. If you frequently move back and forth between devices, the Export and import workspace feature helps: it lets you carry over your labels, stars and notes, without any cloud service involved. :::tip title="Take your organization with you" If you have already added labels, stars and Captain notes on one device, export your workspace and import it on the other device. That way you keep your organization with you locally, without an account or upload. ::: ## Privacy: everything stays on your own device Email archives often contain sensitive information. That is why it matters that Mbox Viewer works 100 percent locally. Your file is read in your browser and stored in the local database on your device (IndexedDB). No emails go to a server, there is no telemetry and the extension requests no network permissions. The only situation where network access comes up is optional: if you want to embed external images in a PDF export. If you want to wipe everything afterwards, you can do so via Settings, with Clear database. :::faq ### Do I need to convert the mbox file for the Mac? No. Mbox is a platform independent format. The same .mbox or .mbx file works on both Windows and macOS. You only move the file, you do not need to convert it. ### Why do some emails look strange after the transfer? Usually this happens because the file passed through a text editor along the way that changed the character set or the line endings. Always move the file unchanged and do not open it in an editor. In Mbox Viewer you can use the Raw tab to see what the real content looks like. ### My mbox file is very large and will not open. What now? Raise the in-memory limit in Settings, or split the archive into smaller mbox files and open them one by one. Also check that the copy is complete by comparing the file size on both systems. ### Is my email uploaded anywhere when I do this? No. Mbox Viewer works entirely locally in your browser. Nothing goes to a server, there is no telemetry and the extension has no network permissions. Your data sits in the local database on your own device. ### Can I take my labels and notes to the other device? Yes. Use the Export workspace feature in Settings on one device and Import workspace on the other. That way your labels, stars and notes come along without a cloud service. ### Does a Google Takeout export work too? Yes. You drag a .mbox.gz file from Google Takeout straight into Mbox Viewer; it is unpacked and read automatically. :::