Re: Why a PC?
Well, it doesn't have to be a PC, of course. To a degree, this is one of those sort of projects that you can do with whatever you happen to have lying around. The first time I tried out OpenBSD (a very long time ago) was when I wanted to press an old SparcStation into use, and it was about the only OS I could find that would support the hardware.
So, if you happen to have a SparcStation sitting in your junk room, or a Raspberry Pi, or a spare old PC, as long as you can get an OS onto it, you can probably get by with that - as I said in last week's part, you don't need a huge amount of oomph to run a mail server.
In this case, I used a x86 system because a) I had previously been using one and b) a new one (the Revo One) turned up to review at just the time I needed it, and is a nice compact bit of kit.
There are some BSD flavours available for the Pi, so you could give it a go with that, and much of the Postfix / Dovecot instructions here will be applicable on other OS flavours too (with key differences in things like how to add packages, or start and stop services).
The one caveat I would mention with regard to less mainstream platforms is that the less popular your platform is, the more likely that you will have to compile some parts of the system yourself, rather than simply downloading pre-compiled packages. That's not exactly a hardship, and the Ports system on BSD makes it pretty easy, but it will make things slower.