# Client-side mbox viewer versus online upload tools ## What is the difference between client-side and online? There are two ways to open and search an old email archive, such as an .mbox or .eml file. The way they handle your data is completely different. A client-side tool runs entirely inside your own web browser, on your own computer. Client-side literally means "on the user's side". You open the tool, drag your file into it, and all the unpacking, reading and searching happens on your own device. Nothing goes to the internet. Mbox Viewer by Cloud Captains works this way: 100 percent local and offline. An online upload tool works the other way around. You pick your archive file, and that file is sent to the company's server. There it gets unpacked, stored and processed. Only then do you see the result in your browser. Your email now sits, sometimes briefly and sometimes longer, on a computer that is not yours. ## Why does this difference matter so much? Email archives are among the most sensitive files you own. An mbox or pst often holds years of messages: private conversations, invoices, password reset emails, contracts, medical appointments, chats with your lawyer or accountant, and attachments containing passport scans or pay slips. The moment you upload an archive like that to an online tool, you hand over control. You no longer know exactly where your mail lives, who can access it, how long it is kept, or whether the server is properly secured. With a client-side viewer none of this applies: the file stays on your own device and you stay in control. :::info title="How Mbox Viewer handles your data" Mbox Viewer stores everything in IndexedDB, a storage area inside your own browser on your own device. The extension's Chrome permissions are empty, there is no telemetry, and no servers are involved. You can wipe everything at once through Settings, Clear database. ::: ## The risks of uploading your archive Uploading to an online tool brings a few concrete risks that are easy to overlook. - A data breach at the provider. If the server gets hacked, your entire email history could leak. You have no influence over that. - An unclear retention period. Many services claim they delete your file "after processing", but you cannot verify whether that actually happens. - Third-party access. Employees, subcontractors or government requests may reach uploaded data, depending on where the server is located. - Transmission across the internet. While uploading, your mail travels over networks you do not control. - Vendor lock-in and business models. Sometimes your data is the product: metadata can be analysed or resold. :::warn title="Take care with sensitive or confidential mail" If the archive contains business correspondence, legal files, medical records or mail covered by privacy law such as the GDPR, do not simply upload it to a random online service. In many cases that is not even allowed without a data processing agreement. A local viewer avoids this problem entirely, because the data never leaves your device. ::: ## Why processing locally is better Processing locally has a few clear advantages over uploading. Your privacy is guaranteed by design, not by a promise. With an online tool you have to trust the privacy policy. With a client-side tool the data simply cannot leak to a server, because it never goes there. It also works offline. Because everything happens in your browser, you do not need an internet connection to read or search your archive. Handy on a train, in a secured environment, or when you just want to be sure nothing goes out. There are no limits set by someone else. Online services often cap the file size or charge money for larger archives. Locally, the only limit is your own device. And you keep control. You decide what you open, what you export and when you wipe everything. :::tip title="External images stay blocked by default" Mbox Viewer does not load external images in messages automatically. That prevents tracking pixels, tiny invisible images a sender can use to see whether and when you open a mail. If you do want to see the images, you load them per message with a button. ::: ## Getting started with a local viewer :::howto title="Open your archive safely and locally" 1. Open Mbox Viewer by Cloud Captains in Chrome. You can find the extension at mbox-viewer.online. 2. Drag your file into the window. Supported formats include .mbox, .mbx, .eml, .emlx, .msg, Maildir folders and .mbox.gz from Google Takeout, which is unpacked automatically. 3. Wait for the archive to load. Everything happens locally in your browser, nothing goes to the internet. 4. View your messages in the list on the left and read them on the right, using the Preview, Raw and Forensic tabs. 5. Search your archive with operators such as from:, subject:, has:attachment or after:2024-01-01. 6. Done? Wipe your data through Settings, Clear database, so nothing is left behind. ::: ## When does an online tool actually make sense? To be fair, an online tool can be handy if you want to quickly share a small, non-sensitive file with someone, or if you cannot or do not want to install software. But as soon as it concerns your own sensitive email archive, the convenience rarely outweighs the privacy risk. A client-side viewer gives you the same convenience without your data ever leaving your device. :::faq ### What exactly does client-side mean? Client-side means the processing happens on your own device, in your own browser, instead of on a server somewhere on the internet. With Mbox Viewer your archive is unpacked, read and searched locally, without anything being uploaded. ### Does my email archive get uploaded anywhere when I use Mbox Viewer? No. Mbox Viewer works 100 percent local and offline. There are no servers, no uploads and no telemetry. Your data is stored in IndexedDB inside your own browser and stays on your device. ### Why should I not just upload my archive to an online viewer? Because you then hand over control of sensitive mail. You do not know where your data lives, who can reach it, how long it is kept or how well the server is secured. In a data breach your entire email history could end up exposed. ### Do I need internet to use Mbox Viewer? No. Because everything happens in your browser, you can open and search your archive offline too. Internet is only optional if you want to embed external images into a PDF export. ### How do I wipe my data when I am finished? Go to Settings and choose Clear database. This removes all loaded messages and related data from your browser's local storage. ### Is a local viewer suitable for confidential or business mail? Yes, especially for that. Because the data does not leave your device, you never have to send an archive with legal, medical or privacy-sensitive information to an unknown server. That makes a client-side approach a safe choice for sensitive correspondence. :::