A possible followup for this article would be to explain how to deal with a distributed configuration.
* You have a machine on Internet not necessary a OS X box, let say a FreedBSD with sendmail and managing your mail under the domain "example.org". The machine is "yeah.example.org"
* You have your OS X box on internet with an ISP (example.net). So the ISP give you a temp.ip.add.example.net.
1st question: How can you configure your local sendmail to deal with the mail you send and being identify as it if it was example.org which sends it.
2nd question: How to configure the pop, imap ? to send the mail of one of the account to the machine "yeah.example.org" to your local machine.
3rd question: You had another machine on your local network a laptop. And you want that the mails between the external server, the OS X box, and the OS X laptop are all synchronised. So you can use them alltogether (It's not a trivial Problem).