No. ☺️

There is no one-word answer to this question. It depends on whether you delete, empty or expunge messages in a folder, e.g. when you empty Trash, messages are permanently removed from Trash, but they are not removed from the original folder until that folder is compacted.

It's worth reading this article carefully and doing a few tests to become aware of exactly how compacting and deleting works.

Thank you for your replies. I forgot to mention I'm using POP. Will have to read the article carefully to understand how it works.