- Windows

How To Terminally Delete Emails In Thunderbird By Compacting Folders

ThunderbirdThunderbird is Mozilla’s desktop eMail client. In my hands Thunderbird has been working smoothly and efficiently for many years, which is reason enough to stick with it.

Did you know that eMails in the Thunderbird eMail client are not terminally deleted?

Even if you empty the trash folder, they are not physically removed, neither from the trash folder, nor from the original folder. Yes, this does mean that you can recover those eMails. However, that will be the subject of another story.

The issue here is that Thunderbird will grow to an enormous size if all the spam and mails you have trashed are never actually removed. In fact, the traces are multiplied if you move mail from one folder to another and this will add to the size of each of these folders.

So let me explain how you can finally and forever get rid of deleted eMails and thereby reduce the size the Thunderbird eMail client takes up on your hard drive.

The process of terminally removing traces of moved or trashed eMails from folders is called compacting. There are two ways to do this and they’re both rather simple.

Automatic Compacting

Per default, automatic compacting is not enabled in Thunderbird. To enable it, go to > Tools > Options > Advanced tab > Disk Space section and check the box next to > “Compact folder when it will save over …” and set a KB value. The default value is 100 kB.

save disk space

Thunderbird will now start compacting folders automatically, whenever it saves a reasonable amount of disk space.


The problem here is that your Thunderbird eMail client may not respond well at random, at moments when the critical value was reached to set off folder compacting.

Besides, you should not do anything other than reading eMails while folders are being compacted. Otherwise folders can get corrupted or you could lose data. For example eMails you were writing when compacting started may refuse to send or save.

In other words, automatic compacting can be very annoying, especially in situations where you need to use it urgently.


There are two solutions. If you’re an experienced user, you can customize folder compacting using the Config Editor. If you can’t be bothered with config editing, you should compact folders manually.

Customize Compacting

Automatic folder compacting is still feasible if you make Thunderbird ask for permission before it starts. That way it’s still automatic and you don’t have to think about it, but you won’t be interrupted for good at random intervals.

To open the Config Editor, go to > Tools > Options > Advanced > General and click the > Config Editor button in the bottom right.

config editor

In the > Filter: line type > mail.purge.ask to see whether the entry exists. If it does, double-click to set the value to > true.

purge mail

If the entry does not exist, right-click on any entry and select > New > Boolean and enter > mail.purge.ask as the preference name. Finally, select > true and click OK.


Manual Compacting

To entirely avoid random disruptions due to automatic folder compacting, do it manually whenever you deleted a bunch of spam or an eMail you want to remove for good.

This process is incredibly simple as well. Just right-click on a folder you wish to compact and select > Compact from the menu.

save disk space

The status bar will display the progress of folder compacting. As stated in the warning above, be sure not to do anything within Thunderbird, while the process is underway!

compact folder

The status bar will also announce when compacting was completed.


For a trash folder I had never compacted and which contained almost four thousand messages, compacting took less than a minute.

If you run into problems, please refer to the comprehensive coverage regarding Compacting Folders on the mozillaZine.

How do you keep your eMail client slim and speedy?

About Tina Sieber

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