Exchange Server known issues in 2026: what's open, what's fixed and how to stay informed

After every Exchange Server security update, the same question comes up: is what I'm seeing a known issue, or something I broke? This roundup lists what Microsoft itself has published for the current Exchange Server SE update, what the latest update fixed, how to confirm each issue on your own servers, and where to keep watching, because these lists change after release.

Symptoms: match what you see to a published issue#

These are Microsoft's own descriptions, quoted from the KB:

The table covers Exchange Server SE RTM. Exchange 2019 and 2016 are past end of support; the Exchange Team's V2 announcement says the updates for those versions are ESU updates only, so check the known issues listed for the exact update you install. If you're still on them, see your end-of-support options.

Why it happens#

Each KB describes its issues in one line. Where Microsoft publishes a cause, workaround or fix, it appears in the KB or on the Exchange Team Blog, so take it from there rather than from forum guesses. Two patterns are worth recognising:

• Hybrid free/busy over Microsoft Graph is where rich coexistence is heading as EWS is retired in Exchange Online, and both free/busy issues above sit in that path. On hybrid deployments, test free/busy after every update.
• A regression introduced by one update can be fixed by a later one. The shared mailbox wrapper messages came with the June 2026 update and are fixed by the September update and its V2, so staying current is part of the fix.

How to fix it#

1. Find the build you're actually on#

Get-ExchangeServer shows the CU level but not security updates. Read the file version of ExSetup.exe on each server instead, and compare it with Microsoft's "Exchange Server build numbers and release dates" page:

Get-ExchangeServer | ForEach-Object {
  Invoke-Command -ComputerName $_.Name -ScriptBlock {
    (Get-Command ExSetup.exe).FileVersionInfo | Select-Object FileVersion } |
  Select-Object PSComputerName, FileVersion }

2. Install the latest update for that build#

For SE RTM that's SU10V2 (KB5129955), which fixes the two resolved issues above. Install it from an elevated prompt, one server at a time, using maintenance mode in a DAG. If setup misbehaves, follow installing Exchange security updates.

3. Confirm an open issue before you work around it#

• Published calendar: request the subscriber's calendar.ics URL from a browser, then search the IIS logs for failures under /owa/calendar/:
  PowerShell

  Select-String -Path 'C:\inetpub\logs\LogFiles\W3SVC1\u_ex*.log' -Pattern '/owa/calendar/' |
    Where-Object { $_.Line -match ' 500 ' } | Select-Object -Last 20

  500s that started after you installed the August 2026 update or a later one match the known issue, which Microsoft lists for the August and September updates and the V2. A 401, 403 or 404, or a calendar that never worked, points to the publishing configuration instead.
• Delegated free/busy: compare a mailbox that has delegates with one that doesn't, in the same location. If every cross-premises lookup fails, it's a general hybrid problem: work through hybrid free/busy troubleshooting.
• ContentEngine deadlock: the KB names an internal component, not the user-facing symptom. Word breakers split text into words for the search index, so look at indexing first. Watch whether the not-indexed count keeps rising, and inspect individual items with the CSS-Exchange Troubleshoot-ModernSearch.ps1 script:
  PowerShell

  Get-MailboxStatistics -Identity user@contoso.com | Format-List BigFunnel*
  Get-MailboxDatabaseCopyStatus * | Format-Table Name, Status, ContentIndexState

  Admins have also reported, in comments on the September 2026 announcement, Outlook and OWA search missing recent mail. The KB doesn't connect those reports to this issue, so treat them as unconfirmed. If an index is Failed or the problem predates the update, see fixing Exchange search.

4. Apply only Microsoft's workaround#

Use the workaround in the KB or the Exchange Team Blog. Removing a security update to make a functional issue go away also removes the fixes for the CVEs it closes. Treat that as a last resort, and take it only on Microsoft's advice.

Verify the fix#

• Every server reports the same ExSetup.exe version, matching the latest SU for its build.
• The Exchange Server Health Checker runs clean. Microsoft's KB recommends running it after installing the update. See fixing what Health Checker finds.
• Cross-premises free/busy shows the right busy times, a test subscription to a published calendar loads, and new mail turns up in Outlook and OWA search.

How to stay informed#

• Exchange Team Blog: each release gets a "Released: month year Exchange Server Security Updates" post, and posts are revised after release. The September 2026 post was updated several times.
• The KB's "Known issues in this update" section: read it before you patch and again a week later.
• Microsoft 365 admin center › Health › Message center for Exchange Online changes that hit hybrid. Recent examples from the Exchange Team Blog: EWS deprecation in Exchange Online started on 1 October 2026 (see EWS switch-off in Exchange Online); the ExchangeOnlineManagement module should be updated to 3.10.1 or newer; and Exchange Online is throttling and blocking mail sent over the OnPremises connector by Exchange 2016/2019 servers below a minimum build (the post calls it the final public update baseline).
• Health Checker after every update, because it also flags required follow-up configuration.

References#

• KB5129955: version 2 of the security update for Exchange Server SE RTM, October 2, 2026 (Microsoft Support)
• KB5121573: security update for Exchange Server SE RTM, August 11, 2026 (Microsoft Support)
• Released: September 2026 Exchange Server Security Updates (Exchange Team Blog)
• Exchange Team Blog (Microsoft Tech Community)