I am very careful before reading emails from unknown sources. Whenever I see an email that looks unfamiliar to me and the source of which I cannot pinpoint, I become extra careful and alert.
Spam emails are not harmful if you do not open them. They are also not that harmful when you already pen them provided that you do not click anywhere inside of them. Spam emails usually contain some links that if clicked can be very bad for your computer or can cost you a lot of money if your personal information happens to leak. This is why I am extra careful every time a slam email appears in my inbox.