Outlook Calendar Not Syncing? Check the Account Type First

Appointments you add in Outlook don’t show up on your phone (or vice versa), or the calendar simply won’t update across devices.

Why: Calendars only sync over an Exchange-based connection (Microsoft 365, Outlook.com, or Exchange ActiveSync). POP and IMAP accounts sync email only — never calendar or contacts. Most “calendar won’t sync” cases come down to the wrong account type or a desync that a refresh clears.

Fix 1: Confirm the account isn’t POP or IMAP

This is the deciding factor — there is no calendar sync on POP/IMAP at all.

1. File → Account Settings → Account Settings → E-mail tab.
2. Look at the Type column. Exchange / Microsoft 365 syncs calendars; POP/IMAP does not.
3. If it’s IMAP/POP and you need calendar sync, the account must be re-added as Exchange / Microsoft 365 (only possible if your provider supports it — Gmail, for example, needs Google’s own sync, not Outlook’s).

Fix 2: Use the same Outlook version everywhere

If multiple PCs open the same calendar, mismatched Outlook versions cause missing or duplicate appointments. Update every client to the same, fully-patched version: File → Office Account → Update Options → Update Now.

Fix 3: Force a fresh sync

For Exchange/365 accounts that have drifted out of sync:

1. Send / Receive → Update Folder (or press F9) to pull the latest calendar data.
2. If it’s still wrong, rule out a corrupt local cache: close Outlook, and let the OST rebuild (see Fix 4).

Fix 4: Rebuild the offline cache (OST)

An Exchange/365 calendar lives on the server; the local .ost is only a cache, so rebuilding it is safe.

1. Close Outlook.
2. Control Panel → Mail → Data Files, note the .ost path, then in File Explorer rename it from .ost to .old.
3. Reopen Outlook — it re-downloads the mailbox and calendar fresh from the server.

Fix 5: Re-check the device side

On a phone/tablet, confirm Calendars sync is turned on for that account in the device’s account settings. If it’s an Outlook mobile app account, removing and re-adding it as Exchange (not IMAP) restores sync.

FAQ

Why do appointments duplicate instead of going missing? Usually a third-party sync tool or two clients writing the same calendar. Use one sync method, keep all Outlook clients on the same version (Fix 2), and accept/decline meetings individually rather than via auto-rules.

My Gmail calendar won’t sync into Outlook — is that this bug? No. Outlook can’t sync a Gmail calendar over IMAP; use Google’s sync tool or add the account through a method that supports calendar sync.

Sources: Microsoft Support — Can’t sync calendar and contacts with my phone or tablet