Corrupted MBOX: how to recover email from Thunderbird and Google Takeout

In the MBOX format, every message in a folder sits in a single text file, one after another. It's the format used by Thunderbird, Apple Mail and Gmail exports through Google Takeout. A single broken separator is enough to make messages vanish or show up merged.

Signs that this is your problem

• A Thunderbird folder looks empty or has fewer messages than before.
• Messages show another message's content stuck at the end.
• The Google Takeout MBOX is too large to open or import.
• Thunderbird says the folder is being processed and never finishes.

Why it happens

• An interruption while Thunderbird was compacting folders.
• An index (.msf) out of step with the MBOX file.
• "From " lines inside an email body mistaken for the start of a new message.
• Huge Takeout files, with tens of gigabytes in a single "All mail" folder.

How to fix it, step by step

1. Close Thunderbird and copy the profile folder (Help > Troubleshooting Information > Profile Folder).
2. Try Thunderbird's free repair: right-click the folder > Properties > Repair Folder. This rebuilds the .msf index.
3. If the messages don't come back, open the copy of the MBOX file (the file with no extension named after the folder) in Neural Mail Repair.
4. Check the recovered messages in the preview and save as MBOX for Thunderbird or as PST for Outlook.

How to keep it from happening again

• Compact folders while Thunderbird is idle and don't close it during compaction.
• Split old email into folders by year.
• Back up the profile folder with Thunderbird closed.

Menu names may vary with your Outlook and Windows version. Before any repair, make a copy of the data file with Outlook closed.

No software recovers 100% of cases. Overwritten parts, empty files and files encrypted by ransomware have nothing left to recover, and the free preview shows you that before you pay.