When emails are downloaded they are not stored as individual. When you see a folder in the Folder Pane, this is actually an mbox file (assuming you Pop mail account or synchronise subscribed imap folders). When you compact your Inbox folder those 'marked as deleted' are removed, thus freeing up space. So it is still taking up space in the Inbox. In reality, it is still in the Inbox as a 'marked as deled' email hidden from view. When you select to delete an email or put it in 'Trash', from your point of view it moves from eg: inbox to 'Trash' After all, Trash is for unwanted items, so after a period of time allowing for rescuing an email deleted in error, they clear this folder to create more space. Some auto empty Trash folder periodically by default and there is nothing you can do about it. If you have an Imap mail account, there may be a different policy used by the server. Uncheck/select as appropriate: 'Empty Trash on exit' Menu Icon > Options > Account Settings > Server Settings for the mail account Tools > Account Settings > Server Settings for the mail account
Trash folder (also known as 'Deleted') can be auto emptied on exit if option is selected. Please create new folders to organise your wanted emails. Trash folder is not the place to store items that you want to keep. The reason for having a 'Trash' folder is to act as a stop gap because sometimes you might delete an email in error and want to rescue it. They are only in the Trash because you selected to 'delete' them or you moved them manually. 'Trash' means 'Rubblish' to throw away discard delete.