I've actually wondered this myself. Why isn't there a good preconfigured drop in solution for this? It would be especially nice if it had a good web front-end that you could log into remotely. How hard would it be to develop something like this? I assume it would be a matter of forking an existing project and adding some custom configs. The hard part would be the web component.
Freedom box has talked and hopefully worked on doing this, but Im not sure if they have gotten as far as improving the interface of a webmail yet.
Still, I do think there exist some very talented grafic designers out there that would enjoy playing around with a webmail interface. Hopefully, articles like this will increase the demand and interest in run-it-your-self webmail packages, so one would do that and not spend all focus on building more CMSes.