Most service providers don't advertise anything they call "smarthost", but most that do email and that have good reputations will know what you're asking for if you contact their sales. I've recommended Linode and Panix before.
Be sure to search for "smarthost" in quotes, because "smarthosting" is a marketing term.
Be sure to search for "smarthost" in quotes, because "smarthosting" is a marketing term.