How to Repair Folders in Mozilla Thunderbird

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When a mailbox seems off in Mozilla Thunderbird and missing messages, for example, a repair can often help.

Unwanted Inbox Zero?

Your Mozilla Thunderbird inbox is suddenly much lighter, and you can't claim any credit? You've deleted a message weeks ago, you're certain, and it still shows up in a folder?

Sometimes, Mozilla Thunderbird folders' appearance loses track of the underlying structure: messages actually present are not shown, or deleted emails still present.

Fortunately, Mozilla Thunderbird can re-build the folder index (which allows the message list to appear faster than if the folder's full contents had to be loaded) and make it reflect what messages you do have.

Repair Folders in Mozilla Thunderbird

To rebuild a Mozilla Thunderbird folder — when emails have disappeared, for instance, or deleted messages still show:

  1. As a precaution, turn off automatic mail checking.
    • This is typically not necessary but avoids a potential cause for conflicts.
  2. Click with the right mouse button on the folder you want to repair in Mozilla Thunderbird.
  3. Select Properties… from the menu that appears.
  4. Go to the General Information tab.
  5. Click Repair Folder.
  6. Now click OK.
    • You do not have to wait for the rebuilding to finish before clicking OK; do wait before doing anything else in Mozilla Thunderbird, though.

Have Mozilla Thunderbird Rebuild Multiple Folders

To have Mozilla Thunderbird repair more folders' indices automatically:

  • Make sure Mozilla Thunderbird is not running.
  • Open your Mozilla Thunderbird profile directory.
  • Go to the desired account's data folder.
    • IMAP accounts are under ImapMail.
    • POP accounts are found under Mail/Local Folders.
  • Locate .msf files corresponding to the folders you want to have rebuilt.
  • Move the .msf files to the trash.
    • Make sure you do not delete the corresponding files without extension.
    • If you see a file called "INBOX" and a file called "INBOX.msf", for instance, delete the "INBOX.msf" file but leave the "INBOX" file in place.
  • Start Mozilla Thunderbird.

Mozilla Thunderbird will rebuild the removed .msf (index) files.