Compacting Folders in Mozilla Thunderbird

What to Know

  • Deleted messages in Thunderbird are not truly deleted but rather marked and hidden. Compacting fully deletes these messages.
  • To compact, select File > Compact Folders. This process may take time if it's been a while since last compacting.
  • Settings for automatic or periodic compacting can be found under Tools > Settings/Options > Advanced > Network & Disk Space.

When you delete a message in Mozilla Thunderbird, it is not fully removed from the mailbox file but rather "marked" as deleted and hidden from display. This can speeds things up, but it also consumes valuable hard disk space. Compacting refers to the process of deleting these marked messages, clearing valuable space in your inbox.

Learn how to compact folders in Mozilla Thunderbird—both manually and automatically via settings.

How to Compact Folders in Mozilla Thunderbird

To reclaim that space for the current account and compact folders in Mozilla Thunderbird, select File > Compact Folders.

If your folders are large and you've deleted lots of messages since the last compacting, this process may take some time.

Have Mozilla Thunderbird Compact Folders Automatically

You can set up Mozilla Thunderbird to free disk space automatically—with or without prompting. Here's how:

  1. From the menu bar, select Tools > Options (Windows). If you're on a Mac or Linux device, select Tools > Preferences.

  2. Select Advanced > Network & Disk Space.

  3. Under Compact folders when it will save over_MB, enter your desired disk space threshold. The default value is 20 MB, which is fine under most circumstances, but if you find Mozilla Thunderbird compacting often, you can increase the trigger to 100 or 200 MB, for example.

  4. Choose Close.

Folder Compacting Options and Settings

To compact folder when you are prompted, select Compact Now under Do you wish to compact all local and offline folders to save disk space?

To make Mozilla Thunderbird compact without prompting in the future, make sure Always ask me before compacting folders automatically is not checked.

To choose whether you will be prompted when Mozilla Thunderbird is about to compact folders automatically:

  1. From the menu bar, select Tools > Options (Windows). If you're on a Mac or Linux device, select Tools > Preferences.

  2. Select Advanced > General > Config Editor. If prompted, select I'll be careful, I promise!

  3. Under Search, type mail.purge.ask

  4. Double-click mail.purge.ask under Preference Name to toggle its setting (visible under Value).

    • False will not prompt you when automatically compacting folders.
    • True will ask you before compacting folders automatically.
  5. Close the about:config preferences window and select Close.

If you find messages missing or deleted emails appearing after compacting your folders, you can try rebuilding or repairing their indices.

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