This talk introduces the only open source Outlook addin, Outlook CalDavSynchronizer, to synchronize calendars, tasks and contacts with CalDAV andCardDAV servers, Google or iCloud services. We will focus on the challengesfaced integrating those protocols into Outlook. A case study of a largedeployment in an university infrastructure will show the benefits of such asolution. Microsoft Exchange/Outlook together with the Exchange ActiveSync (EAS)protocol for mobile clients is still the most widely used groupware solutionin enterprise environments. But what open source alternatives exist asExchange replacements out there? Actually, there are many productsimplementing the open CalDAV and CardDAV protocols like SOGo, Horde Groupware,Zarafa or Owncloud/Nextcloud. Client integration is possible with mobile iOSand Android clients and desktop clients like Thunderbird/Lightning, butOutlook users were kept in the dark or needed proprietary 3rd partyconnectors. In 2015 even Zarafa, which had a strong focus on seamless Outlookintegration, announced the discontinuation of their connector. A promisingalternative, SOGo together with OpenChange as a native Open Source MAPIimplementation, is not yet production ready in large scale enterprisescenarios and development stalled lately. Luckily, there is another solutionon the horizon. This talk introduces the only open source Outlook addin,Outlook CalDav Synchronizer, to synchronize calendars, tasks and contacts withCalDAV and CardDAV servers, Google or iCloud services. We will focus on thechallenges faced integrating those protocols into Outlook. Together with otheropen source solutions like SOGo or Nextcloud and DAVDroid for Android, a fullopen source exchange server replacement for desktop and mobile clients isfinally possible and enterprise ready. A case study of a large deployment inan university infrastructure will show the benefits of such a solution.