how can I move mails from the “sent” folder to a local folder and have it there not deleted when the original message is deleted from trash?
Subject: Sent is a VIEW
Sent is a view, not a folder.
Views show documents based on criteria, and the Sent view shows all documents in the database that you have sent.
So you cannot “remove” documents from it.
There’s nothing preventing you from also adding that document to one or more folders, so it will show in multiple views and folders.
Remember that folders are just a list of pointers to documents in the database. The document is not really stored inside the folder.