Email Sync Problems

Fix Microsoft To Do Crashing When Syncing Flagged Outlook Emails

Natalie Taylor Oct 20, 2026 868 views

Question details

The user needs to resolve an issue where Microsoft To Do crashes or becomes unresponsive when attempting to sync a large volume of flagged emails from Microsoft Outlook.

Fix Microsoft To Do Crashing When Syncing Flagged Outlook Emails

Product: Microsoft To Do / Microsoft Outlook
Device & OS: not provided
Scenario: Synchronizing thousands of flagged Outlook emails as tasks into the Microsoft To Do application.
Observed behavior: Microsoft To Do crashes or becomes completely unresponsive. Furthermore, removing the flags in Outlook does not immediately remove the items from To Do, indicating a significant delay or caching issue with Exchange synchronization.

Before you start

Check your Exchange account connection status within Microsoft Outlook to ensure it is currently online and fully connected to the server before attempting to clear flags or force a synchronization.

Solution 1Recommended

Unflag Emails in Outlook and Allow Time for Synchronization

Removing the flags directly from Outlook and allowing the server time to process the changes can clear the backlog that causes To Do to crash.

Because Microsoft To Do relies heavily on Exchange server synchronization, attempting to process thousands of flagged emails at once can overwhelm the app's cache. Clearing these flags at the source is necessary.

Locate flagged emails in Outlook

Open Microsoft Outlook and navigate to your inbox or the specific search folder where you keep mass-flagged emails.

Clear the flags

Select the flagged emails you no longer need as tasks, right-click the selection, choose 'Follow Up', and then click 'Clear Flag'.

Wait for server sync

Keep Outlook open and connected to your Exchange server. Do not open Microsoft To Do immediately; allow a few hours for the backend synchronization to process the removed flags.

Unflag Emails in Outlook and Allow Time for Synchronization

Synchronization Delay: Due to server caching, it is normal for cleared flags to still appear in Microsoft To Do temporarily until the Exchange synchronization fully completes.

Solution 2

Use Outlook Tasks as a Workaround

If Microsoft To Do remains unresponsive due to corrupted cache or overwhelming data, use the built-in Outlook Tasks feature which handles large volumes of Exchange data more reliably.

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Frequently Asked Questions

Why does Microsoft To Do crash when syncing flagged emails?

Microsoft To Do can become overwhelmed and crash if there is an unusually large backlog of tasks—such as thousands of flagged emails—attempting to synchronize from Microsoft Exchange all at once, which exceeds the app's memory or cache limits.

How long does it take for unflagged Outlook emails to disappear from Microsoft To Do?

Synchronization times vary based on Exchange server load and your network connection. It can take anywhere from a few minutes to several hours for heavily cached data to clear from Microsoft To Do after unflagging items in Outlook.

Can I manage my Exchange tasks without using Microsoft To Do?

Yes, you can use the classic Outlook Tasks feature built directly into the Microsoft Outlook desktop client. It is often much more stable for managing large volumes of flagged emails linked to an Exchange account.

Will uninstalling and reinstalling Microsoft To Do fix the sync crash?

Reinstalling the app may clear corrupted local cache data, but if the issue stems from thousands of pending flags on the Exchange server, the crash will likely reoccur upon logging back in until the flags are properly cleared in Outlook.

Outlook & Email Problems