Here are two projects to look at: Citadel, (http://www.citadel.org http://www.citadel.org) -- A lot of features, but the UI is somewhat dated. It would be inte

Here are two projects to look at: Citadel, (http://www.citadel.org http://www.citadel.org) -- A lot of features, but the UI is somewhat dated. It would be interesting to see a revival. OpenXchange, (https://www.open-xchange.com https://www.open-xchange.com) -- Commercial project with an open source core. Much nicer UI, I'd be worried about long term support if/when the original company stops developing it.