Note: try to avoid downloading e-mail while compacting folders, as it's been reported that this can cause corruption or a 'folder being processed' error. If you have trouble doing this and the process stalls, try compacting one folder at a time: click on the folder and choose 'Compact This Folder'. Compacting an account may take a few seconds or a few minutes, depending on how much mail you have and how recently you've compacted folders. To compact all folders in an account: click on the account on the left, and then click 'File -> Compact Folders'.

The main difference is that most other email clients by default automatically compact the folder when a certain amount of space is wasted. It isn't some unique feature of Thunderbird. If you don't compact folders your mail folders can grow very large and the program can behave erratically, so it's a good idea to do it periodically.ĭon't confuse compacting folders with compressing files. These hidden e-mails remain in the folder until it is compacted. Even emptying the trash does not permanently delete them.

When you delete or move e-mails from a folder, Thunderbird and other e-mail programs really only 'hide' them ( i.e., mark them as ready for permanent deletion), and these hidden e-mails still actually remain in the folder.

Following is an excerpt from the Thunderbird knowledge base article and instructions for compacting folders.

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