Using an offline folder (a folder that resides only on your PC) can be useful for storing e-mail to free up more space on the server where the e-mail resides, for example.
You can create as many folders as you need.
Step 1
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Click your right mouse button on "Local Folders" and choose "New Folder"

Geef de folder een naam die je handig vindt.
Step 2
You can now easily drag and drop emails into this folder but you can also move or copy several emails at once.
To do this, select the mails you want to move or copy and then right-click on them.
Then select "Move to > Local Folder > e-mail backup" or the name you chose for your folder.

Where do I find the local Thunderbird folder?
In Thunderbird, go to the local folder whose location you want to know and right-click on it :

The field you then see contains the path to the folder you are looking for.

Copy this and paste it into your explorer without the bit after "Local Folders"

The files you see in that folder contain your mail files. It's a good idea to back them up regularly, preferably automatically.