A couple of things to consider when making the decision to host your sites yourself:
1) The way e-Smith is designed you will not have user1@domain1.com be different than user1@domain2.com (Each mail account will have a unique username).
2) You can make the mail relay from the outside world with some tweaking.
3) You are using RedHat already with e-Smith, just a well customised version.
I have set up e-Smith for multilple domains, and have no problems. I use a another customised RedHat v7.1 for my hosting, both work well in their respective environments.