I'm curious why you didn't base this on Kolab [0]? It looks like you share a lot of common components [1], but they've already done the integration work and added additional features.
[0] http://kolab.org/ [1] http://kolab.org/sites/kolab.org/files/u51/KolabServer-Compo... vs https://mailinabox.email/static/architecture.svg