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How long does it typically take for a calendar invite to sync to a shared mailbox in Outlook

Dave 0 Reputation points
2026-01-14T00:35:15.2066667+00:00

We are using Outlook 365 web, and are experiencing delay in syncing calendar invites to shared mailboxes. The sender of the calendar invite and the receiver are both under the same tenant/domain. As part of troubleshooting, we tried sending to 3 different shared mailboxes (all under the same domain) and all experience delay issues (SM1: 4 hours delay, SM2: 8 hours delay, SM3: 3 hours delay). Even a newly created shared mailbox does not help as it also experience delay syncing its calendar.

Already reached out to Microsoft support and they are saying that this is the default expected behaviour for shared mailboxes. We want to know if its true, if yes, then how long does it typically take for a calendar invite to sync to a shared mailbox in Outlook? Thanks.

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  1. Steven-N 25,305 Reputation points Microsoft External Staff Moderator
    2026-01-14T13:36:16.9733333+00:00

    Hi Dave

    Thank you for reaching out to Microsoft Q&A forum

    As far as I know, under normal conditions, calendar invites sent to a shared mailbox should appear on the calendar almost immediately or within a few minutes. Microsoft’s shared calendar improvements are designed to support near real-time syncing for internal users. Delays of several hours are not consistent with this expected behavior.

    However, your testing showed delays of 3, 4, and even 8 hours before invites appeared on various shared mailboxes. This is far outside the norm for same-domain invites, indicating something unusual. In fact, other users have reported similar symptoms as a problem scenario. In your situation, since even a newly created shared mailbox had the issue, we can likely rule out mailbox-specific problem and it suggests a systemic factor.

    In my opinion, the most likely scenario is that your shared mailbox calendars are being viewed/updated under the old synchronization method (perhaps due to how they were added in Outlook Web or Outlook Desktop). This could cause reliance on periodic refresh cycles and lead to the unusually long delays you observed, as states in Calendar sharing in Microsoft 365

    User's image

    Therefore, I recommend the following approach, you can try them to see if the behavior can be resolved:

    1. Ensure the shared calendar is using the “new model” of calendar sharing, which supports instant syncing. This requires the calendar to be explicitly shared and accepted using a modern Outlook client.
    2. Avoid accessing the shared mailbox calendar via legacy methods, as these may rely on older sync mechanisms that use periodic polling rather than push-based updates.
    3. Verify that the shared mailbox is configured with AutomateProcessing set to AutoUpdate. This ensures that meeting invites are automatically added to the calendar as tentative without manual intervention. You can check this using PowerShell: 
         Get-CalendarProcessing <SharedMailboxAlias> | Select AutomateProcessing

    Note: If the shared mailbox is primarily used for scheduling, consider converting it to a resource mailbox (room or equipment), which supports AutoAccept and processes invites immediately upon receipt.

    You can try the above approach, if the problem still persists, kindly let me know in the comments for further support.

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