I have been running this setup for a few years now and it's great. I used to have a problem with GMail rejecting a lot of forwarded emails as spam but I set up graylisting (http://postgrey.schweikert.ch/) and since then I haven't had a single email rejected. It probably also matters how long you've been running your server so I would expect (assuming you're not actually sending or forwarding spam) to see the number of rejected emails decrease over time.