[lutusp]

> I know this is true for mass marketing emails, but is it the same for transactional emails also?

That depends -- have you ever signed up with the sender? If so, you are asking to be spammed. When a company requires a signup, they are breaking the law, therefore you must not sign up. By signing up, you make the spam legitimate.

If they send you an unsolicited e-mail, they are criminals. If they require a signup to opt out, they are criminals. But if you sign up, they aren't. That's why they want you to sign up.