Mac Outlook - syncing to M365 issues

Kevin McGuire 20 Reputation points
2026-04-12T21:14:33.0666667+00:00

Hi,

I'm attempting to help an M365 user who is experiencing issues with Outlook not properly syncing Mail & Calendar. It's shown up as:

  • Sent emails do not appear in the sent folder.
  • Incoming mail does not appear in Outlook.
  • Calendar invites do not update.
  • Changes to meetings to which the user has been invited don't appear in calendar.

Have checked Outlook in iPhone and Outlook OWA....emails and calendar appear in both locations as expected.

I've deleted and re-added the user's M365 account in Mac Outlook a few times. Previously un-sync'd items appear correctly, but as time goes on, the same issue appears with new items.

Currently, the user works exclusively in a home environment with WiFi from ISP. No other apps on this Mac experience the same issues.

Any suggestions?

Thanks!

Outlook | MacOS | New Outlook for Mac | For business
Answer accepted by question author
Hendrix-C 19,985 Reputation points Microsoft External Staff Moderator
2026-04-12T22:26:44.12+00:00

Hi @Kevin McGuire,

Thank you for your detailed sharing.

Based on your information, this is highly a sync problem on that user Outlook client since the mailbox is working normally in Outlook OWA and on iPhone app. The most common reason of this behavior is due to corrupted local Outlook cache which can cause synchronization problems.

To clear cache in Outlook for Mac, you need to open New Outlook > on the Outlook toolbar, select Tools > Accounts > select the account you want to reset > select Manage option (or the three dot icon) > Reset account

For further assistance, please share which Outlook and macOS version so I can investigate further about the situation.

Please understand that my initial response does not always resolve the issue immediately. However, you can try these steps and let me know if it works. If not, we can work together to find a solution.

If you have any questions or need further assistance, please feel free to share them in the comments on this post so I can continue to support you.    

Thank you for your understanding and cooperation. I'm looking forward to your reply.

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  1. AI answer

    2026-04-12T21:14:48.99+00:00

    Use the following targeted checks and fixes on Outlook for Mac, focusing on sync and connectivity, since OWA and iPhone are working correctly (server-side is healthy):

    1. Confirm Outlook is online and connected
    • In Outlook for Mac, on the Outlook menu, ensure Work Offline is not checked.
    • On the Tools ribbon, select Accounts and check the Exchange/Microsoft 365 account in the left pane:
      • If the status icon is orange, Outlook is not properly connected to Exchange.
      • If this occurs intermittently, it indicates a client-side connectivity or profile issue rather than a server issue.
    1. Verify general network connectivity
    • From the Mac, use a browser to confirm normal Internet access and that Outlook on the web works for the same user.
    • If only Outlook for Mac has issues while other apps and OWA are fine, proceed with Outlook-specific remediation below.
    1. Clear the Outlook cache for affected folders Corrupted local cache can cause symptoms like missing sent items, missing new mail, and calendar not updating even though OWA is correct.

    For each affected Exchange folder (Inbox, Sent Items, Calendar):

    1. Ensure the Mac is online and connected to Exchange.
    2. In Outlook’s navigation pane, Ctrl+click or right‑click the Exchange folder (for example, Inbox or Sent Items) and select Properties.
    3. On the General tab, choose Empty Cache.
    4. Wait while Outlook re-downloads items from the server.

    Caution: Emptying the cache removes any content not yet synchronized to the server (including items like contacts’ mail certificates). Ensure important data is synced or backed up before clearing.

    1. Recreate the profile only after cache clear Since the account has already been removed and re-added multiple times, focus on cache corruption first. If issues persist after emptying caches and confirming connectivity, then:
    • Remove the account again from Tools → Accounts.
    • Add the Microsoft 365 account back and allow full resync.
    1. Check for known sync issues and client version
    • There are known cases where Outlook for Mac intermittently stops syncing and asks to remove/add the account again. The current workaround is to:
      1. Open Outlook on the web.
      2. Go to the People pane.
      3. Under the user’s mailbox, select Favorites.
      4. Delete all Favorites contacts.
    • Also ensure Outlook for Mac is fully updated, as some sync issues (including those related to Conditional Access) have been fixed in recent builds.
    1. If Conditional Access is in use
    • If the tenant uses Conditional Access (CAE) policies and the Outlook for Mac build is older than 16.92 (Build 24120731), Outlook can fail to sync mail and calendar.
    • Update Outlook for Mac to at least version 16.92 so the CAE-related sync issue is resolved.
    1. If issues continue
    • If, after cache clearing, profile recreation, and client update, the Mac client still intermittently fails while OWA and mobile remain healthy, treat it as a client bug or tenant-side issue and open a support ticket so engineering can review backend logs for that mailbox/tenant.

    References:

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