Many users use their email software's Preview Pane to read their emails, (I do), and that effectively opens the email and allows any html or images to be displayed. Either of those can send the message back to the sender.
The difficulty is, to delete an email you have to highlight it first, and that displays it in the Preview Pane. If your Inbox is empty and the first email you receive is spam, then that may display automatically in the Preview Pane.
I use a variety of options to reduce spam.
1] My first email in my Inbox is a 'Test' email I have sent to myself. It always sits there. Any other email that arrives is listed below that.
2] My Thunderbird email software has Junk controls that sift out a lot of spam and junk. Fortunately I can click the Junk button for an email without highlighting the email.
3] I have also set up Message Rules in the Tools menu. I will add to this as necessary, but it automatically removes emails with certain keywords, eg the usual sexual limb enlargement email, Rolex, Viagra, and so on. These emails are sent to Trash and deleted from the server. The Trash folder is emptied when I exit Thunderbird.
4] Any suspect email, but I am unsure, I remove the Preview Pane, (View > Layout > Preview pane), highlight the email, then goto View > Message Source to see the whole email message, which includes email routes, and allows the main body to be seen in raw format.
5] If I receive a whole batch of junk mail, I will Select All, then use CTRL to de-select the ones I want to keep, then delete the rest.