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Exchange Server Subscription Edition · Release Health

Known issues in Exchange Server Subscription Edition

Problems Microsoft has confirmed for this version, from its official release health notes. 4 still open or mitigated.

Exchange Server: SE 2019 2016
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Active known issues

Open & Mitigated
4 active issues
  1. Open Exchange Server SE Affects: Organizations and IT administrators Started with KB5129955 · Oct 2, 2026

    A ContentEngine deadlock occurs because of missing Korean WordBreaker rule files

    The KB lists a deadlock linked to missing Korean WordBreaker files. Search results might be missing, mail delivery might be delayed, or Outlook might stall or disconnect.

    Workaround status No workaround yet

    Microsoft has not published a manual workaround for this issue yet. Defer KB5129955 on affected systems until a fix is released.

  2. Open Exchange Server SE Affects: Organizations and IT administrators Started with KB5121573 · Aug 11, 2026

    Availability (free/busy) fails for delegated mailboxes in Exchange hybrid deployments using Graph API only

    In some Exchange hybrid setups, checking a delegated mailbox's availability might show an error or no details. This affects Graph API-only setups; Microsoft says the service fix is available.

    How to apply the workaround Exchange Management Shell

    The override enables the Graph API route for availability checks. Refreshing Variant Configuration applies the change immediately.

    Step-by-step instructions, command & download kit
    3 steps Exchange Management Shell Instructions (.txt)
    Show steps & kit Hide steps & kit
    Step-by-step procedure
    1. Check the current value of the override.
    2. Only if it is Enabled=False or was previously disabled, set it to Enabled=True.
    3. Run the refresh command to apply the change immediately.
    Exchange Management Shell 
    > Get-SettingOverride -Identity EnableRouteThroughMSGraphFeature> Set-SettingOverride -Identity EnableRouteThroughMSGraphFeature -Parameters "Enabled=True"> Get-ExchangeDiagnosticInfo -Process Microsoft.Exchange.Directory.TopologyService -Component VariantConfiguration -Argument Refresh
  3. Open Exchange Server 2016 2019 SE Affects: Organizations and IT administrators Started with KB5121573 · Aug 11, 2026

    Published calendar (.ics) returns HTTP 500 for calendar applications

    After KB5121573, subscriptions to anonymously published Exchange calendars might stop refreshing. Calendar apps might receive an HTTP 500 error.

    How to apply the workaround IIS / URL Rewrite

    Add a URL Rewrite rule to the Exchange Back End site. It adds layout=premium to calendar-feed requests that do not already have a layout parameter.

    Step-by-step workaround instructions
    5 steps
    Show steps Hide steps
    Step-by-step procedure
    1. Confirm that the IIS URL Rewrite module is installed.
    2. In IIS Manager, add a blank inbound rule to the Exchange Back End site's URL Rewrite settings.
    3. Match ^owa/calendar/.+\.ics$ and add a condition that excludes requests whose query string already contains layout=.
    4. Rewrite the URL to {R:0}?layout=premium&{QUERY_STRING}; clear Append query string.
    5. Apply the rule on every server that serves the published calendar. No application pool recycle is required.
  4. Open Exchange Server 2016 2019 SE Affects: Organizations and IT administrators Started with KB5103212 · Jul 14, 2026

    Wrapper messages appear in shared mailbox in hybrid environments after installing the June 2026 Security Update

    In Exchange hybrid, sent messages might appear as attachments in an Exchange Online shared mailbox Inbox. With an on-premises sender, Send As or Send on Behalf rights, and message-copy enabled, this occurs; KB5121608 resolves it.

    How to apply the workaround Exchange Management Shell

    The article says to remove the override if it was created as a workaround for this issue.

    Step-by-step instructions, command & download kit
    2 steps Exchange Management Shell Instructions (.txt)
    Show steps & kit Hide steps & kit
    Step-by-step procedure
    1. In Exchange Management Shell, check whether the override exists.
    2. If it exists, remove it.
    Exchange Management Shell 
    > Get-SettingOverride "DisableBlockSharedAndUserMailboxHeaders"> Remove-SettingOverride "DisableBlockSharedAndUserMailboxHeaders"

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