Yeah, the book is full of it 😉
Seriously, though, it does come with a CD of all the code. As you can imagine, something like Hotmail doesn't scale well to posting on a forum like this...
The PHP imap functions can handle POP3 servers as well, so one would write the server interface with those. User auth module might not be too difficult; it'd need db, of course. User management interfaces with db and SMTP server.
Simple in concept, I suppose; lots of coding to make it work. If I didn't want Squirrelmail or some such, I'd buy the book and start with their code....