When I open my email inbox, why have my emails already been opened or read? But not by me.Thank you.

Everytime I access my mail almost all have already been opened, like they have been opened and read, but I hav'nt done it, it's a bit annoying as most of it is spam and I am trying to stop them by marking them as spam. Still every time I receive mail like half an hour ago 10 email and 9 already opened so the spam that has been opened which I am trying to stop think they have correct email are now going to send even more spam email. Can anyone help?  THANK YOU.