From MozillaZine Knowledge Base
- This article was written for Thunderbird but also applies to Mozilla Suite / SeaMonkey (though some menu sequences may differ).
You try to open the Inbox and nothing happens (the message list stays blank). The "Building summary file" message (in Thunderbird's status bar) lasts for minutes. Sometimes the contents of the Inbox appear and sometimes they don't. Thunderbird hasn't entirely frozen; it still responds but the Inbox doesn't display messages or behaves strangely.
This usually happens to very large Inbox files that haven't been compacted.
- Exit Thunderbird.
- Make several copies of the file "Inbox" in safe places (outside your profile folder).
- Rename the file "Inbox" to "InboxOld".
- Delete the file "Inbox.msf".
- Start Thunderbird. It will create a new, empty Inbox.
- There should be a new folder InboxOld visible in the folder pane. Right-click it and select "Compact This Folder".
- If this succeeds, try to open it.
- If you can open it without problems, you are done and you can delete the copies (after you make very sure).
If you can't open it without serious problems:
- Exit Thunderbird.
- Delete the files "InboxOld" and "InboxOld.msf" and the folder "InboxOld.sbd" from your profile folder.
- Download the ImportExportTool extension. Note: The ImportExportTool extension used to be called the mboximport extension.
- Start Thunderbird and install the ImportExportTool extension.
- After restarting Thunderbird, try to import one of the copies using the ImportExportTool extension.
- If this succeeds, you can move the messages to where you want them (I suggest somewhere else than the Inbox).
- If you get a message that the file is not in a valid mbox format, you can try opening the file using any text editor (note that for big Inbox files - I was working with one problematic one that was more than 200MB - it will take a long time to open the file)
- Check that the opened text file is of a proper mbox format. In the case of my problematic file, the corruption occurs in the first line of the file. Correcting the line results in successful importing using the ImportExportTool extension.
- If this also fails, you may have to take your loss.
In the future:
- Regularly compact your folders.
- Keep your Inbox empty by moving messages you want to keep to other folders or child folders.