Look... I like the techy tutorial. This is the right direction. It's come up in the forums recently about doing this very thing, especially whenever a mail host forgets to renew their DNS etc.
But.... a million HOWTO's for this are a two-second Google away, using the exact same software and configuration, and aren't split across four pages, and don't spend the first entire article telling you to install an OS and then install a package.
It's a good article, the right direction, but I just don't feel that you have targetted the right audience here at all. Any beginner would look at those submissiond lines and go "ARGH!". Anyone who's ever done this themselves knows that line by heart. The others can follow the same HOWTOs across the net that provide the same steps.
Postgrey is also such an obvious next step that - guess what - every HOWTO covers it as well.
Though I welcome the article, and may be being unnecessarily harsh, what are you going to cover once you've exhausted the HOWTO that we can all find on the net, that will keep us reading?