REALLY deleting mail attachments

I am aware that this might/has been explored here in other threads, I just don't find or get it.

I have local (local, not on IMAP servers) mailboxes with a size of 53 GB. Tens of thousands of emails. Many of them with unnecessary attachments, especially if they sit in the trash (which I cannot delete).

When I choose the menu command "remove attachments" the link is removed but the attachment itself remains in the mail folder in my library.