Resolving Dynamics 365 Contact Sync Issues in Outlook – ExchangeSyncIdMapping Overload

Introduction

Recently, our project encountered a critical issue where Outlook crashed for 4000+ users when accessing the contact section. Upon investigation, we found that data in the ExchangeSyncIdMapping entity table had surged dramatically—reaching 800GB to 1TB, severely impacting performance and stability.

Microsoft confirmed that the activation of Outlook filters for individual users triggered mailbox sync between CRM and Outlook, leading to this excessive data buildup. Below, we outline our journey to identifying the root cause and applying solutions to mitigate the issue.

Initial Findings: Outlook Filters Causing Sync Issues

Upon raising a Microsoft support ticket, we discovered that the CRM sync process was automatically applying Outlook filters when users onboarded. Microsoft initially advised updating or deleting these filters to prevent future sync issues.

We followed their recommendation using XRM Toolbox, based on Microsoft’s official documentation: 🔗 Configure Default Sync Filters

However, after deleting these filters, the sync issue persisted, and ExchangeSyncIdMapping entity space continued growing. This led us to further investigate its underlying cause.

Further Investigation: Security Role & Mailbox Customization Impact

Upon deeper analysis, we found that the issue originated when new users were assigned security roles and their mailboxes were enabled through a custom plugin. This automatically created Outlook filters, causing continuous synchronization—even after filters were deleted.

Additionally, data in the ExchangeSyncIdMapping entity was not decreasing despite efforts to stop the sync.

Microsoft’s Recommended Fix & Implementation Steps

To fully resolve the issue, Microsoft provided the following steps:

Step-by-Step Resolution:

1. Create a new view with a filter excluding contact data to prevent sync from CRM to Outlook.
2. Import this view into the CRM solution.
3. Use XRM Toolbox to set the new view as default and delete/deactivate the existing view causing the issue.
4. Disable the mailbox for affected users.
5. Apply the new view to all existing and new users using XRM Toolbox.
6. Manually delete all Outlook contacts for impacted users.
7. Reactivate mailboxes, which will trigger a cleanup and reduce ExchangeSyncIdMapping entity data.

Final Thoughts & Lessons Learned

Through this process, we uncovered how hidden automation during user onboarding can significantly affect data synchronization.

• Proactively reviewing and controlling default sync filters is crucial for preventing Outlook-related crashes and data overload.
• Customization via plugins should be closely monitored, especially when impacting system-wide configurations.

Following Microsoft’s guidance, we successfully reduced ExchangeSyncIdMapping entity data and stabilized the synchronization process.

If you are facing similar CRM-to-Outlook sync issues, consider these best practices early to avoid excessive data buildup!

This version enhances readability, breaks down complex information, and adds clear headings for structured flow. Let me know if you'd like further tweaks, or if you want to highlight any additional points