I'm not sure to what extent this qualifies as an MS Exchange replacement (it certainly isn't a drop-in replacement), but the Open-Xchange suite, which powers ma

I'm not sure to what extent this qualifies as an MS Exchange replacement (it certainly isn't a drop-in replacement), but the Open-Xchange suite, which powers mailbox.org, is quite good: https://en.wikipedia.org/wiki/Open-Xchange https://en.wikipedia.org/wiki/Open-Xchange