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MSF file is the “Mail Summary File,” which doesn’t contain the actual data but only the summary of the emails. If there’s a problem with this file, like if it is corrupted or has become too large, Thunderbird may start behaving erractically. It might constantly become unresponsive or freeze or crash.

MSF file, How It Damaged

And in most cases, *.msf (message summary file) gets damaged or become too large due to not using the “compacting” feature of Thunderbird. Compacting is a way to reduce file size and unnecessary items from the database. It does not compress anything (like in .zip files), but deletes items like trash folder and so on. It’s a good way to maintain the performance for Thunderbird and also to lower the risk of data corruption and crash. Due to not doing it regularly, the most commonly, it can damage MSF file.

If your Thunderbird is unresponsive quite often, there is a likely chance that this is the cause.

One of the quick way to solve this is by deleting the MSF file and then re-launching Thunderbird, which forces the client to create a new MSF file from scratch.

Here’s how to fix corrupt MSF file:

  1. Go to your Thunderbird’s Profile folder (the Profile for which you are facing the issue). You can go to C:\Users\<Windows user name>\AppData\Roaming\Thunderbird\Profiles\<Profile name>\.
  2. In latest versions of Thunderbird, you can also to Help -> Troubleshooting Information -> then click “about:profiles” -> and then click “Open folder” for the corresponding Profile.how to fix corrupt msf
  3. Find the folder that contains the email data. For example, if you are using the Gmail IMAP, there will be a folder named imap.gmail.com. For local folders, there will simple be a folder named “Mail.”
  4. Select the MSF files and delete them

Now you can start Thunderbird and it will re-create the MSF file and download all the content (mail summary) again.

Do the same steps for MSF files in other Profiles as well, if Thunderbird is causing issues in all those profiles as well.

Conclusion

Note that this fix is only if the MSF file is too large or very mildly damaged. If there’s a bigger corruption issue, you might need to repair the entire database or rebuild your profile again (which includes downloading the mailbox folders from the mail servers).

It’s also a good habit to compact your folders regulary. In latest versions of Thunderbird, there is even default setting for automatic compacting but you can modify to your own liking and also do it manually whenever you think there is a need to do so.