While configuring, setting up and administering a mail server can be a fun (for some) and good learning opportunity, it's generally way more work then it's worth. I say this as someone who has spent a lot of time doing just that with various *nix based mail packages.
Dealing with spam, access lists, anti-virus/malware filtering, certs, etc. is an ongoing job. Not to mention, as many are noticing, dealing with blacklists. Most companies outsource this to third party providers (Google, Hosted Exchange, MXLogic, etc.). And the average user who might only need a handful of email addresses for themselves, family and friends can often find much better options for free or close to it.
GApps for Business used to offer a free version, but they killed it a year or so ago. I still have three or four accounts though, and use one quite extensively, and was lucky to be grandfathered in.
Yes but when you run a forum people register and need to get that email to verify there account info. If nobody get's the email then your dead. Not sure how I would do that using a 3d party.