What is the Exchange Online Synchronisation Log?

In Exchange Online (part of Microsoft 365), the synchronisation log is a file or record that keeps track of issues when Outlook (the client) is trying to sync with Exchange Online (the server).

• • Where it appears:
    In Outlook, you may see a folder called “Sync Issues” with subfolders like:
    • • Conflicts
      • Local Failures
      • Server Failures
  • Purpose:
    The synchronisation log stores messages and reports when Outlook can’t properly sync an item (email, calendar, contact, etc.) between the local OST file on your computer and your Exchange Online mailbox.
  • When it happens:
    • • Network problems (poor connectivity).
      • Mailbox throttling or temporary issues in Exchange Online.
      • Large attachments or corrupted items.
      • Add-ins or antivirus interfering with Outlook.
      • A mismatch between the local cached copy and the server copy.
  • What you’ll see:
    Often, messages titled something like “Synchronization Log” or “Synchronization Errors” appear in those folders. They look like emails but are actually system-generated reports.
  • Impact:
    Most of the time, these logs are harmless and can be ignored. But if you or your users notice missing items (e.g., a calendar invite not showing up), the sync logs help identify which items failed.

In short:
The Exchange Online Synchronisation Log is a diagnostic report inside Outlook that records sync issues between your local Outlook data and your Exchange Online mailbox. It helps troubleshoot when Outlook and Exchange are “out of sync.”

Where to find Synchronisation Logs in Outlook

1. 1. Open Outlook (desktop app).
      This only applies if you’re using Outlook in Cached Exchange Mode (with an OST file).
   2. Go to the Folder Pane.
      Scroll down until you see a folder named Sync Issues.
      Inside, you’ll find three subfolders:
      • • Conflicts – When the local copy of an item conflicts with the server copy.
        • Local Failures – Items Outlook couldn’t upload to the server.
        • Server Failures – Items downloaded incorrectly or not available from Exchange.
   3. Look for “Synchronization Log” messages.
      These are system-generated reports. They often look like emails with subject lines such as “Synchronization Log: Local Failures”.

How to interpret them

• • Conflicts folder
    • • Example: You edited a calendar meeting offline, but the organizer changed it online.
      • Action: Decide which copy to keep. Outlook may keep both.
  • Local Failures folder
    • • Example: An email with a huge attachment never left your Outbox.
      • Action: Delete/retry the item.
  • Server Failures folder
    • • Example: Outlook downloaded a corrupted item that doesn’t exist on the server anymore.
      • Action: Usually safe to ignore unless you’re missing important mail.
  • Synchronization Log emails
    • • They summarize what failed to sync.
      • Often look cryptic (with item IDs and folder names). They’re mainly for troubleshooting by admins.

When to worry

• • If users notice missing emails, calendar events, or contacts.
  • If Outlook keeps prompting with “disconnected” or “trying to connect to server.”
  • If the Sync Issues folder grows very large (hundreds of MBs) – this can cause OST corruption.

How to fix common sync problems

• • Make sure Outlook is online and the network is stable.
  • Delete and re-add the Outlook profile (creates a fresh OST).
  • Run Outlook /safe mode to test for add-in conflicts.
  • Check mailbox size and clean up old items.
  • As a last resort: disable/re-enable Cached Exchange Mode.

👉 In everyday use:
Most users never need to check the Sync Issues folder. It’s mainly useful for admins or when a user complains that “something is missing.”

📝 Exchange Online Sync Log Troubleshooting Cheat-Sheet

⚡ Tip for admins: If you want to monitor recurring problems, you can enable Outlook logging (File → Options → Advanced → Enable troubleshooting logging) and then analyze the logs, but for most cases the Sync Issues folders are enough.