A few years ago, in Vienna, we founded grommunio. With the help of many tools, from Dovecot to Postfix and many more, a revolutionary open-source groupware was born. On top of the classic mail server stack, we implemented APIs and protocols of Microsoft's Exchange (and later MS 365 server) in free, opensource versions, one by one. This way, we built a "drop-in"-replacement for MS Exchange that supports unmodified, plugin-free clients with Outlook, Android, Apple and Web clients out of the box. It comes with chat, video conferences, file sync, administration, domain, user and account management and much more. Most recently, we implemented Exchange Web Services. In this talk, we will have a look at how we re-implemented the protocols and how it works with clients. We'll talk about problems, solutions and quirks of our work and the challenges of both supporting many older versions and following Microsoft's newest developments. Our support for Microsoft Exchange (and other) protocols follows the protocol specifications of EWS, EAS (Exchange ActiveSync), OXCRPC/OXCROPS, MAPI/HTTP and others.