I did read the article and that's exactly what he's doing. He forwards everything to Gmail, and then allows users to send from Gmail via his SMTP server (or maybe doing some spoofing but it wouldn't look so clean). Essentially outsourcing hosting of inboxes (but just inboxes) to Gmail.

I run my own mail server, but most of my employees use Gmail. So I have forwarding set up: employee@dnalounge.com simply forwards to employee@gmail.com. When sending mail using their Gmail account, they set their From line to employee@dnalounge.com. (Google lets you do this if you jump through some hoops to verify that you can actually receive mail at that account.)