Office 365 and CalDAV: How to Sync Your CalDAV Calendar with Outlook

If you're running a CalDAV calendar server and want to sync it with Microsoft 365 or Outlook, you've probably discovered that Office 365 doesn't support CalDAV. Unlike some other calendar platforms, Microsoft has never implemented the CalDAV protocol in its cloud services. Here's what that means and what your options are.

Does Outlook support CalDAV?

No. Neither Microsoft 365 (formerly Office 365) nor any version of Outlook supports CalDAV natively. Microsoft uses its own protocols:

• Exchange Web Services (EWS) — the legacy API
• Microsoft Graph API — the modern REST API for Microsoft 365
• Exchange ActiveSync (EAS) — for mobile devices

CalDAV (RFC 4791) is an open internet standard used by Nextcloud, SOGo, Radicale, Baïkal, Posteo, and many other calendar servers. But Microsoft has chosen not to implement it.

This means there is no way to add a CalDAV server URL to Outlook or Microsoft 365 directly. You can't subscribe to a CalDAV calendar the way you can with an ICS feed — and even ICS feeds in Outlook are read-only and delayed.

What about the Outlook CalDAV Synchronizer?

The Outlook CalDAV Synchronizer has been a popular workaround for years. It's an open-source COM Add-in for Outlook Desktop that bridges CalDAV and Outlook by syncing events locally.

However, since Microsoft started rolling out the new Outlook for Windows in 2024, this solution has become problematic:

• The new Outlook does not support COM Add-ins — the CalDAV Synchronizer simply cannot be installed
• Microsoft is replacing classic Outlook with the new version via Windows Update
• There is no migration path for COM Add-ins to the new Outlook architecture

For a detailed comparison, see our post on Outlook CalDAV Synchronizer alternatives.

What about Exchange ActiveSync?

Some CalDAV servers — notably SOGo — offer ActiveSync support. This works with Outlook Desktop but has limitations:

• ActiveSync syncs with the local Outlook profile, not with the Microsoft 365 cloud calendar
• Events synced via ActiveSync are not visible in Outlook Web, Outlook mobile, or to other Microsoft 365 users
• It doesn't work with the new Outlook, which relies entirely on the Microsoft 365 cloud

ActiveSync is useful in some scenarios but doesn't provide cloud-to-cloud calendar synchronization.

The solution: CalDAVconnect

CalDAVconnect solves the Office 365 CalDAV problem by syncing your CalDAV server with Microsoft 365 through the Microsoft Graph API. The sync happens server-to-server in the cloud — no client-side software needed.

How it works

1. CalDAVconnect connects to your CalDAV server (Nextcloud, SOGo, Radicale, Baïkal, etc.)
2. It connects to your Microsoft 365 account via OAuth (Graph API)
3. Changes are synced bidirectionally — events created, edited, or deleted on either side are reflected on the other within seconds

Why cloud-to-cloud matters

Because CalDAVconnect syncs directly to your Microsoft 365 cloud calendar, the synced events are available everywhere:

• New Outlook (Windows) — fully supported
• Classic Outlook Desktop — fully supported
• Outlook Web (outlook.office.com) — fully supported
• Outlook mobile (iOS / Android) — fully supported
• Microsoft Teams — calendar integration works seamlessly

This is a fundamental advantage over the Outlook CalDAV Synchronizer and ActiveSync, which only sync to a local profile.

Sync speed

• Changes in Microsoft 365 → CalDAV server: 1–2 seconds (via Microsoft Graph subscriptions)
• Changes in CalDAV server → Microsoft 365: 7–8 seconds average (via CTag-based polling)

Getting started

1. Sign up for CalDAVconnect (free during the public beta)
2. Add your CalDAV server credentials
3. Connect your Microsoft 365 account via OAuth
4. Select which calendars to sync
5. Your first sync runs automatically

Setup takes about 5 minutes. No software to install on any device.

Privacy and compliance

CalDAVconnect is hosted on Hetzner in Nuremberg, Germany. All data is processed within the EU and encrypted at rest. Fully GDPR compliant — relevant for organizations that need to maintain data sovereignty while syncing with Microsoft 365.

Conclusion

Office 365 doesn't support CalDAV and likely never will. The Outlook CalDAV Synchronizer is no longer viable for new Outlook users. CalDAVconnect provides a modern, cloud-based alternative: bidirectional sync between any CalDAV server and Microsoft 365, available on every device, syncing in seconds. Try it free during the beta.