Outlook CalDav Synchronizer, a two-way bridge from Outlook to any DAV server

Outlook has no native CalDAV, so the add-in does the translating

Microsoft Outlook has never spoken CalDAV or CardDAV natively, which is the gap this project fills. It is an Outlook plugin, a COM add-in loaded inside the mail client, that synchronizes events, tasks and contacts between Outlook and Google, SOGo, Horde or any other CalDAV or CardDAV server. The stated audience is a person or organisation whose data lives on a hosted DAV server, whether that is a Nextcloud instance, a Google account, or a mail provider, and who does not want to abandon the Outlook calendar and contact windows to see it. Supported Outlook versions are 2021, 2019, 2016, 2013, 2010 and 2007, plus Office 365 for Desktop, so the plugin spans a long tail of client versions rather than only the current one. The project started in 2015 as a master thesis at the University of Applied Sciences Technikum Wien and is now maintained by Generalize-IT Solutions OG in Vienna.

Mapping every field the add-in can reach, and some it cannot

The feature list reads like a data dictionary of Outlook, and it is the best evidence for how the synchronization works. The add-in syncs reminders, categories, recurrences with exceptions, importance, transparency, the organizer and attendee fields including your own response status, and it supports tasks as well as appointments. Contacts are handled two ways: CardDAV on the generic path, and a Google native Contacts API path that maps Google contact groups onto Outlook categories, with a matching Google Tasklists path that syncs through the Google Task API into Outlook task folders. Distribution lists are a specific headache, and Outlook lists are mapped onto SOGo VLIST, KIND:GROUP or iCloud group formats. Server colours map onto Outlook category colours, and Outlook category colours map back onto the server, with per-event color handling under RFC 7986 and a WebDAV Collection Sync mode under RFC 6578. The plugin can also create missing collections on the server with MKCOL, and it can write an Outlook formatted RTFBody out as an HTML description through the X-ALT-DESC attribute.

One solution, four ways to trigger it

Synchronization can be started by time, by change, or by hand, and the add-in keeps a report of the last sync runs and its status, which matters more than it sounds once a two-way sync starts making changes you did not type. A system tray icon carries notifications. Calendars and address books are auto-discovered, the sync range is configurable, and a single profile can sync multiple calendars, with category filtering available to route CalDAV calendars or tasks into particular Outlook categories. The repository layout shows how the work is divided: a CalDavSynchronizer/ directory holds the Outlook-facing side, a GenSync/ directory holds a general synchronization engine with its own GenSync.UnitTests/ project, and separate OAuth projects exist for Google and for Swisscom. GlobalOptions.xml sits at the root as a shared settings file. The Outlook-facing code is therefore a layer on top of a generic engine, which is the shape you would want if you had to point the same mechanism at something other than Outlook.

Installing it takes a zip file and a runtime check

Installation is a desktop installer rather than a package manager entry, and the README is specific about it. From release 4.0.0 onward, .NET framework 4.8 is the minimum requirement. The archive you download is named with the version in it, and setup.exe is what you run.

OutlookCalDavSynchronizer-<Version>.zip

setup.exe

You can change the default install path, but it has to sit on the C drive. From version 2.9.0 the default location is the following path, and the installer remembers the chosen directory so that later updates land in the same place.

ProgramFilesDir\CalDavSynchronizer\

If the installer complains about a missing Visual Studio 2010 Tools for Office Runtime, the README says to install it manually, and to update it to version 10.0.60912 if an older one is present, because some COMExceptions have been fixed in that version. That runtime is the piece most likely to trip up a new install.

10.0.60912

The server list is long, and the client list is short

Thirty-three CalDAV servers are listed as tested, from Baikal and Cozy Cloud through DAViCal, FastMail, Google Calendar, iCloud, Kolab, Nextcloud, Owncloud, Posteo, Radicale, SabreDAV, SOGo, Synology DSM, Yahoo, Yandex, Zimbra 8.5 and Zoho Calendar. That breadth is the practical answer to the question of whether an unusual server will work. The parallel on the other side of the synchronization is the recommended Android client: the project works with DAVx5 and tests interoperability with it, and calls it the recommended DAV client, so the same server can be reached from a phone with a purpose-built client while Outlook holds the desktop copy. One caveat sits in the same paragraph: per-event coloring by mapping the Outlook category color to the COLOR attribute of the event is described as experimental support, so that particular mapping is the least settled part of an otherwise settled feature set.

Version 4.7.0 changed the security posture of the parser

The changelog records 4.7.0 on 2026-08-10 as a security release that hardens XML parsing logic to avoid XML External Entity (XXE) injection, alongside updated NuGet packages, a new Postassl profile, and a fix for properly quoting a display name when an email address syntax is used to specify an attendee or organizer. Version 4.7.1 followed on 2026-08-17 with bug fixes for empty address strings in attendee mapping, tracked as gh issue 478 and 479 and SourceForge 2181 and 2179. Before that, 4.6.0 on 2025-06-04 added an Aruba profile and updated NuGet packages. The relevant point for an operator is that the XXE hardening landed in a version, and an instance still running an earlier release is running a parser that was later changed for that reason. The project is not archived and the last push was on 2026-08-17, the same day as the 4.7.1 tag.

Where the add-in is the wrong choice

There is a hard floor under the product and it is written in the install instructions. .NET 4.8 is not supported on Windows XP, Windows 8 or Windows Vista, and the README offers a backport to .NET 4.0 for Windows XP in a separate forked project rather than in this repository. Outlook 2007 can only install the add-in for the current user, while Outlook 2010 and higher can install it for everyone on the machine, which is a real constraint for shared workstations. Beyond the platform limits, the shape of the product is the limitation: this is a synchronization bridge for people who want their data inside Outlook. If the requirement is to reach a DAV server without Outlook, a dedicated client such as DAVx5 does the job without a COM add-in, a mail client, or a Windows desktop. The add-in is also the heavier option in another sense, because an AGPL component running inside a mail client inherits the obligations of that licence, and what that means for a modified version in a commercial deployment is a question for a lawyer rather than for this article.

Editorial conclusion

Outlook CalDav Synchronizer fits an organisation that keeps its calendars and contacts on a CalDAV or CardDAV server but wants to work inside Outlook rather than a browser, and it fits a user who wants the free path in a category where the README calls this the only free Outlook CalDAV plugin. It does not fit a team that wants its data to live anywhere other than Outlook, and it does not fit a machine still on Windows Vista, Windows 8 or Windows XP without taking the separate backport. Check three things before deploying it: that the target machine has .NET 4.8 and the Visual Studio 2010 Tools for Office Runtime at version 10.0.60912, that the server is on the tested list, and that your organisation accepts AGPL-3.0 for a component that runs inside a mail client.

Frequently asked questions

Official sources

1. aluxnimm/outlookcaldavsynchronizer on GitHub
2. Issues
3. License: AGPL-3.0
4. README
5. Releases