I've been considering this, my main problem is that my cash flow is fairly spotty at times for a dedicated server and I would prefer not to have to worry about the security of my box, uptime and the like.
A guide I mentioned earlier (http://flurdy.com/docs/postfix/index.html) shows how to set this up on Amazon EC2. If you haven't partaken, Amazon is still giving away a free-for-a-year tier of EC2.
However if I must, I will.