I don't know about the GP, but I used the "How To Install iRedMail On Ubuntu 12.04 x64"[1] guide on Digital Ocean's community site and was pretty successful in setting up my mail server, complete with postfix, dovecot, spamassasin, greylisting, ssl/tls, webmail, etc.

1. https://www.digitalocean.com/community/articles/how-to-insta...

I wrote it up the other day conveniently. Here you go:

https://news.ycombinator.com/item?id=6750630

Basically, mutt, postfix, procmail.