How to apply automatic message filters to subfolders, too?

To make filtering works for subfolders: create a new string preference "mail.server.default.applyIncomingFilters" to "true". Even if it seems odd, do NOT create a boolean setting, it has to be a STRING setting with the value "true".

Source: http://forums.mozillazine.org/viewtopic.php?f=39&t=1444045&p=12768821#p12768821 see reply of "geoyo1"

According to [1], set up Thunderbird to run filters on subfolders:

1. Click the "Tools" menu
2. Click the "Options" sub-menu
3. Select the "Advanced" and "General" tab
4. Click the "Config Editor..." button
5. [Optional] Click the "I'll be careful (...)" button
6. Right click on top of any of the preferences shown and click "New >"
7. Select "String"
8. Preference name: mail.server.default.applyIncomingFilters
9. Preference value: true

Now the second problem is that the filters only work when you click the sub-folder. This seems to be because filters are only executed when new messages are downloaded and that only happens when you click the sub-folder.

According to [2], set up Thunderbird to download messages whenever new mail is detected:

1. Go to the Config Editor (if needed, see above how to get to the "Config Editor..." button)
2. Set mail.imap.use_status_for_biff to false
3. Set mail.server.default.autosync_offline_stores to true

As pointed out in [2], please see the following observations

The advantage of the AutoSync settings is that (...) any downloading is not delayed until you go offline.

(...)

If you use it, be careful about how you configure Thunderbird to check for new mail, to avoid it downloading folders that you didn't expect.

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[1] http://forums.mozillazine.org/viewtopic.php?f=39&t=1444045&p=12768821#p12768821

[2] http://kb.mozillazine.org/Offline_folders#AutoSync