Sync TrekMail Calendar with Microsoft Outlook (Caveats)

Microsoft Outlook does not speak CalDAV. This is a long-running gap on Microsoft's side — the protocol is supported by Apple, Google, Mozilla, and effectively every other major calendar platform, but Outlook has never shipped native support. Connecting Outlook to TrekMail's calendar therefore requires either a third-party plugin or a workaround.

This page covers both, in the order most people end up choosing.

Why this is awkward

When you set up an Outlook profile against a TrekMail mailbox, Outlook reads email over IMAP and sends over SMTP — that part works perfectly. But Outlook's calendar component only talks to its own native sources (Microsoft Exchange, Microsoft 365, and a partial bridge to Google Calendar via OAuth). Without intervention, accepting a meeting invitation in Outlook stores the event in a local PST file on the computer only, and the event never reaches a phone, a tablet, or another desktop.

There are two reliable ways around this.

Option 1 — Outlook CalDAV Synchronizer plugin (recommended)

Outlook CalDAV Synchronizer is a free, open-source Outlook plugin that adds proper two-way CalDAV/CardDAV support. It's actively maintained, widely used, and is the closest thing to a native solution.

Install

1. Download the latest release from https://caldavsynchronizer.org/.
2. Close Outlook completely.
3. Run the installer.
4. Reopen Outlook. You'll see a new CalDav Synchronizer ribbon tab.

Configure

1. In Outlook, click the CalDav Synchronizer ribbon → Synchronization Profiles.
2. Click Add new profile → choose Generic CalDAV/CardDAV.
3. Fill in:
   • Profile name: TrekMail
   • Outlook folder: choose your default Calendar folder (or create a dedicated one)
   • DAV URL: https://trekmail.net/dav/
   • Username: your full TrekMail email address
   • Password: your mailbox password
4. Click Test or discover settings — the plugin will find your TrekMail calendar.
5. Save and close. The first sync starts automatically.

Repeat the steps for Contacts with Generic CalDAV/CardDAV + Outlook Contacts folder + same DAV URL. The plugin distinguishes calendar vs. addressbook by the folder type.

Option 2 — Use Google Calendar as a relay (workaround, no plugin)

If you can't install plugins on your work machine, you can still get sync by adding a Google account to Outlook as a calendar-only account while keeping TrekMail as the email account. Meetings you accept in Outlook then land in Google Calendar, which already syncs to your phone.

1. In Outlook: File → Add Account.
2. Enter your Gmail address and sign in.
3. When Outlook asks what to sync, choose Calendar only.
4. Once added, set the Google calendar as your default calendar: File → Account Settings → Data Files → select the Google calendar → Set as Default.

Accepted meetings now land in Google Calendar, which syncs to Android, iPhone, and the web.

This option doesn't connect Outlook to TrekMail — it routes around the problem. Choose it only if a plugin install is impossible.

Option 3 — Use TrekMail webmail directly for calendar

Forget about Outlook for the calendar side entirely. Keep Outlook for email, and use TrekMail webmail (in your browser) for calendar. Every other phone or desktop you connect via the Apple, iPhone/iPad, or Android guides will sync with the webmail calendar through standard CalDAV.

What does NOT work

• Outlook on Mac with iCloud Calendar bridging — does not pass through to a generic CalDAV server like TrekMail. It only links Outlook to iCloud's specific Apple servers.
• Outlook for Web (outlook.com) — does not allow adding external CalDAV accounts at all.

Common problems with the plugin

"Authentication failed." Confirm you're using your mailbox password (not the TrekMail dashboard password) and that the DAV URL is exactly https://trekmail.net/dav/.

Plugin disabled after Outlook update. Outlook sometimes auto-disables COM add-ins after major updates. Re-enable it under File → Options → Add-Ins → COM Add-Ins → Go.

Events missing categories or formatting. Outlook's data model is richer than VCALENDAR — some category labels and rich descriptions don't survive the round-trip. The plugin documents this on its issue tracker.