Intertwingularity is not generally acknowledged — people keep pretending they can make things deeply hierarchical, categorizable and sequential when they can’t. Everything is deeply intertwingled.”
— Ted Nelson

Exuding the point, the devlopers of ZOE have released this epochal application to the open source community. (recently updated in fact. ) In their own words…

Zoë is a email client. It’s also a email server. And a long term archive. And a search engine. And an application server. All that at once on your desktop. Or server. Or both. Or it doesn’t matter because client and server are the same.

It doesn’t get any better than this. And it works on mac, or not!