Usually, if you haven't set anything else up in account settings, when you delete a mail it is '''copied''' to Trash and marked as to be deleted in Inbox and will not be shown anymore. It's easy to restore if needed. As folders are one single file with mail stacked after each other in that file. All to be deleted mails are still intact within that file. To clean up it must be compacted. Then the inbox (or other) will be rewritten into a new file without the deleted. If you delete 100 mails every day and don't compact, that file will be huge. Same thing and even worse if you have IMAP and Trash on server. As you then have to correspond with the server.