IMAP is a protocol with clear specifications on how it works. And so using imap literally means software implementing it will have the same message on the server that are presented to the local computer. End of story. https://support.mozilla.org/en-US/questions/new/thunderbird

To accomplish an approximation of what you want your only choice is to copy the messages you want to keep to Local Folders before deleting from the imap server.

You explained that copying to Locla Folders is the best course of action (if not the only). That leads me to ask how I can accomplish that. I've evaluated how I envision archiving my mail off-line and came to the following:

- I want to automatically copy all email from my imap account to local folders each time Thunderbird syncs with the server - I want to keep the existing folder structure; meaning sent email are copied to a local "sent" folder. Trash to a local "trash" folder, inbox to a local inbox folder etc. - I also want to download and store attachments to local folders - If a new (sub) folder is created on imap, i want to automatically create that same folder in my Local Folders - After copying all mail and attachments to local folders I want to delete any mail older than 100 days from my IMAP but preserve them locally. - If something gets deleted of my IMAP after it was copied over to Local Folders it is not deleted from Local Folders. So If I deleted a submap on my IMAP the submap is not deleted from Local Folders.

How can I achieve this? I studied mail filters but I see no ways to copy from a certain (sub)folder. Nor do I see ways to detect a new sun folder and create it locally.

Do I need plugins to achieve my desired result? And if yes, which plugins? Of course I had a look myself but I found nothing about creating folders automatically, and most references to plugins are many years old leading my to worry that these plugins might be outdated or buggy.

Actually he did not say copying to local folders was the best course of action, it is not. Using POP for local storage is the best approach because it is designed to be stand alone from the server.

IMAP is synchronized IMAP is not. POP has just a maildrop which is general considered to be the inbox and recognizes no folders, IMAP does. Those are the realities. Automated archiving is possible specifying a local folder 5to archive to, but really I would not recommend it. Nothing that copies/moves large numbers of emails from an IMAP server/account to local folders can really be considered to be a reliable process.