It is technically easily possible to fake the sender's address when sending mail.

So if I want, I can send with any email address as sender's address. Of course, when someone replies or when the mail can't be delivered, these answers go to the mailers address.

So if I chose your email address as sender's address and send an email to someone else, that someone might think the email came from you...

Of course, though I know how to do that, I won't do it.