I haven't seen anything online, but Peachpit Press has a 10.7 Server Essentials book you can purchase. It's aimed at those looking to get an ACTC and chapter 8 a great section on setting up 10.7 as a Mail Server with webmail.
Sorry I couldn't be more help. Maybe someone else has a good online guide. If you have a more specific question please let me know.
I was just about to make a thread asking about this. I actually have the Peachpit Press Lion Server Essentials book, and it's completely unhelpful. The part about email server is totally a high level overview and lacks useful specifics.