Sounds like a good idea. But it's quite a niche your aiming at (most people that are on-the-go, or are away, have an adequate alternative).

Plus the good old spammers out there would have their fun surely?

I suppose you could have an account system, where you sign up and validate your email, prior to going away, then all you do is sign in and send the emails you need to.

But then technically you're now just another free mail service, defeating the purpose of not having to sign up! Hmmm....