Microsoft Exchange/Outlook together with the Exchange ActiveSync (EAS) protocol for mobile clients is still the most widely used groupware solution in enterprise environments. But what open source alternatives exist as Exchange replacements out there? Actually, there are many products implementing the open CalDAV and CardDAV protocols like SOGo, Horde Groupware, Zarafa or Owncloud/Nextcloud. Client integration is possible with mobile iOS and Android clients and desktop clients like Thunderbird/Lightning, but Outlook users were kept in the dark or needed proprietary 3rd party connectors. In 2015 even Zarafa, which had a strong focus on seamless Outlook integration, announced the discontinuation of their connector. A promising alternative, SOGo together with OpenChange as a native Open Source MAPI implementation, is not yet production ready in large scale enterprise scenarios and development stalled lately. Luckily, there is another solution on the horizon. This talk introduces the only open source Outlook addin, Outlook CalDav Synchronizer, to synchronize calendars, tasks and contacts with CalDAV and CardDAV servers, Google or iCloud services. We will focus on the challenges faced integrating those protocols into Outlook. Together with other open source solutions like SOGo or Nextcloud and DAVDroid for Android, a full open source exchange server replacement for desktop and mobile clients is finally possible and enterprise ready. A case study of a large deployment in an university infrastructure will show the benefits of such a solution.