--- title: "A Thunderbird MBOX and a Gmail MBOX Are Not the Same File" description: "Both are valid MBOX and both open in the same reader, but they are organized on opposite principles: folders versus labels, one file per folder versus one file for everything, and deleted mail that is still in there." url: https://mboxviewerpro.com/blog/thunderbird-mbox-vs-gmail-mbox/ language: en updated: 2026-09-23 source: Mbox Viewer --- [All posts](https://mboxviewerpro.com/blog/) `mbox` `thunderbird` `gmail` # A Thunderbird MBOX and a Gmail MBOX Are Not the Same File Both are valid MBOX and both open in the same reader, but they are organized on opposite principles: folders versus labels, one file per folder versus one file for everything, and deleted mail that is still in there. [David Carrero](https://mboxviewerpro.com/author/) · September 23, 2026 Both are standard [MBOX](https://mboxviewerpro.com/blog/what-is-an-mbox-file/) and both open in the same reader. What differs is everything around the format: Thunderbird gives you one file per folder, Gmail gives you one file with everything in it; Thunderbird organizes by folders, Gmail by labels stored in a header; and a Thunderbird file usually still contains mail its owner deleted years ago. Knowing which one you are holding changes what you should expect to find. ## One file each, or one file total Thunderbird keeps **one MBOX per folder**: `Inbox`, `Sent`, `Archives`, each a separate extension-less file [inside the profile](https://mboxviewerpro.com/blog/where-thunderbird-keeps-your-email/), with a `.msf` index beside it and subfolders in a matching `.sbd` directory. To have “all the mail”, you need all the files. Google Takeout gives you **one file with everything selected in it** — inbox, sent, spam and trash together — split into several parts only when it is very large. Nothing is missing, but nothing is separated either. The practical consequence: if someone hands you “the Thunderbird archive” and it is one file, you probably have one folder of it. That is a common and expensive misunderstanding when the missing folder is `Sent`. ## Folders versus labels Thunderbird’s folders are exclusive: a message is in one, the way a piece of paper is in one drawer. Gmail’s [labels](https://mboxviewerpro.com/glossary/gmail-labels/) are not. A message can be in Inbox, starred and tagged `Receipts` at once, and Sent, Spam and Trash are themselves labels. Takeout encodes this in a non-standard `X-Gmail-Labels` header — a comma-separated list on each message — so each message appears once, carrying all its labels with it. A reader that ignores that header shows you every message and loses the entire organization: twenty years of mail as one undifferentiated list. [Mbox Viewer](https://mboxviewerpro.com/features/) reads it and rebuilds the labels in the sidebar with counts, and `label:` works in [search](https://mboxviewerpro.com/guides/search-syntax-cheat-sheet/) alongside `from:` and the rest. ## The deleted mail that is still there This one surprises people, and it matters legally as often as it matters technically. Thunderbird does not remove a deleted message. It flags it in `X-Mozilla-Status` and moves on; the bytes stay until someone [compacts](https://mboxviewerpro.com/glossary/compacting/) the folder. An external reader has no reason to honour a Mozilla-specific flag, so it shows what is in the file — which can include mail the owner is certain they deleted in 2018. Depending on the situation, that is either a recovery you are grateful for or something to handle carefully before the file goes anywhere. Either way, decide deliberately: compact first if those messages should not travel, or keep them knowingly. And count the messages afterwards, so you know what you actually have. A Takeout export has no equivalent: it contains what was in the account at export time, including whatever was in Trash if you asked for it. ## Counting them will not agree Consequences of all of the above, when you try to reconcile two archives of “the same” mail: - **Thunderbird counts high** if the folders were never compacted — deleted messages are still counted by a reader that walks the file. - **Gmail counts differently by design**, because a conversation you think of as one thing is several messages, each with its own labels. - **Sent mail lives in different places.** A folder in Thunderbird; a label in Gmail, mixed into the same file. If you need one number, merge the sources into a single mailbox and deduplicate by [Message-ID](https://mboxviewerpro.com/glossary/message-id/). That collapses the copies of a message that exists in both archives, which is the only way the arithmetic ever comes out. ## Which one ages better Both, honestly — they are the same open format, and that is the point of it. But they fail differently. A Thunderbird profile is fragile in a specific way: the mail is in files whose names mean nothing outside Thunderbird, in a folder with a random name, and the `.sbd` structure is easy to break by tidying. Copy the whole `Mail/Local Folders/` directory, not the files that look interesting. A Takeout export is fragile in the other direction: it is a snapshot of a day, complete and self-contained, and immediately out of date. It is also the only one of the two that [keeps working after the account is closed](https://mboxviewerpro.com/blog/gmail-account-closed-takeout-still-opens/). The habit worth having is the same for both: keep the original, name it so it means something in ten years, and open it once to check it is what you think it is. --- *Google and Google Takeout are trademarks of Google LLC.* ## Open your archive with Mbox Viewer Native Mac and Windows app. Streams MBOX and EML files of any size, fully offline. [Mac App Store](https://apps.apple.com/app/mbox-viewer-pro/id6759237715) [Microsoft Store](https://apps.microsoft.com/store/detail/9NW3GVFG7DDB)