I haven't played with the service like the other user, so I'm not sure whether this idea is viable.

You could send some sort of initial opt-in email to the recipient that would let them verify their desire to use your service. This combined with usual email validation should be good to keep spammers out.

Ya I think a initial optin would be the easiest/best way to do this.

I would argue that having that flow would also require a database and heavily reduce the usefulness for new users / experimenters.