If you are in the US, then it has to be opt-in. It's the CAN-SPAM law.
If your email list isn't opt-in or double opt-in ("prior affirmative consent"), include a clear notice that states the email is an advertisement or solicitation in commercial messages (section 5(a)(5)(A)(i)). If your list is opt-in or double opt-in, you're exempt from this provision.
But I still think there is room for an honest service with good intentions.
There are plenty of ways to prevent people from abusing such a system that is in discussion here.
But it does look like it is legal to email someone who hasn't asked for the email as long as you follow the rules about disclosure.
http://business.ftc.gov/documents/bus61-can-spam-act-Complia...