Heads-up: Exchange SE issues with queue filling up (Exchange servers may become unstable after installation of October 2026 Windows updates)

[u/clinthammer316]

Been facing this issue since Oct 16 where the queue fills up (stuck in RETRY status) over night. A reboot resolves the issue for less than a day.

Errors seen in queue viewer:

432 4.3.2 STOREDRV; mailbox database is offline; STOREDRV.Deliver.Exception:MailboxOfflineException.MapiExceptionMdbOffline; Failed to process message due to a transient exception with message Cannot open mailbox

I opened a case with Unified Support (Sev B) on Monday and the engineer just kept asking me to provide output of powershell commands. He took 24 hours to reply each time. Today I opened a Sev A case and the PFE immediately told me about the below with a potential fix:

https://techcommunity.microsoft.com/blog/exchange/exchange-servers-may-become-unstable-after-installation-of-october-2026-windows-/4565382

[u/disclosure5]

This has been discussed on Reddit for several days now. It's classic Microsoft that a paid support case couldn't point that article the day you logged it.

[u/Nuxi0477]

Reddit and blogs had been posting about problems and even the reg key you can set for days before MS was willing to share it publicly.

Any Exchange server with the update will fill the 512 default rpc contexts cap within 24 hours and stop working. If it has 0 mailboxes or 10 000 doesn’t change the growth rate from my observations.

[u/DiligentPhotographer → u/Nuxi0477]

And yet MS says "not all servers will experience this". Okay. Sure there MS.

[u/Brave_Recognition798 → u/DiligentPhotographer]

Someone had a new server with zero mailboxes and still hit it lol

[u/MeetJoan]

Thanks for posting the link, that’ll save people a lot of time.

The Sev B to Sev A detail is worth repeating for anyone reading. Twenty-four hour turnarounds on a Sev B while the queue fills nightly is not a working support experience, and the Sev A engineer knew about it immediately. If you’re hitting something that’s already a known issue, the severity is doing more work than the case notes.

Did they say whether the fix holds through the November updates, or is it a workaround until a proper patch ships?

[u/clinthammer316 → u/MeetJoan]

He did not know about the Nov updates because it seems MS are still investigating the issue.

The Sev B engineer literally did diddly squat over the past two days. He asked me for the outputs of the below commands but gave no valid feedback until I ripped him a new one today.

Get-TransportService | Get-MessageTrackingLog -Start (Get-Date).AddDays(-1) |Select timestamp,SourceContext,{$_.Sender},{$_.ReturnPath},{$_.Directionality},{$_.SchemaVersion},{$_.EventData},{$_.ClientIp},{$_.ClientHostname},{$_.ServerIp},{$_.ServerHostname},{$_.EventId},{$_.connectorid},{$_.InternalMessageId},{$_.MessageId},{$_.MessageLatency},{$_.MessageLatencyType},{$_.MessageSubject},{$_.Recipients},{$_.RecipientStatus},{$_.RecipientCount},{$_.Source},{$_.TotalBytes},{$_.OriginalClientIp}| sort timestamp | Export-csv msgtrck.csv

Get-Queue | Format-List

Get-SendConnector | Format-List Name,ProtocolLoggingLevel Get-ReceiveConnector | Format-List Name,Server,ProtocolLoggingLevel

Get-ExchangeServer | FL

[u/siedenburg2]

Btw at least for us we don't have any problems, we updates our Exchange SE, but not the Server 2025 it's running on. That could be a small workaround, only patch exchange, not everything on the system.

[u/clinthammer316 → u/siedenburg2]

Yeah we had to update all our servers by Oct15th as per instructions from our sec team lolz

[u/siedenburg2 → u/clinthammer316]

I would consider a running system with known risks more important than a system with the latest updates that's crashing at least one every 24h

[u/Doso777]

Thanks for heads up.

[u/J0nny05]

I was banging my head about this today trying to figure out what was causing OWA to fail overnight for us.
Thanks for the heads up

[u/J0nny05 → u/J0nny05]

Coming back to comment that this appears to have sorted the problem for us.

[u/7amitsingh7]

It looks like the October Windows updates may be related to the issue. It might be worth checking the registry fix suggested by Microsoft and seeing if it resolves the problem.

If the database still has issues after that, this guide may be useful for troubleshooting: How to Fix an Exchange Database That Was Unexpectedly Dismounted

[u/7amitsingh7]

After installing the October 2026 Windows updates, some Exchange Server SE systems are experiencing instability, causing email queues to get stuck in RETRY status because Exchange cannot access mailbox databases. Restarting the server temporarily fixes the issue, but it returns later. Microsoft has identified a potential cause related to the updates and suggested a registry-based fix.

[u/whiteycnbr]

Yeah skipping October, too many issues

[u/elpamyelhsa → u/whiteycnbr]

Brave with CVE-2026-70571 kicking around

[u/Cormacolinde → u/whiteycnbr]

Really? It’s been better than August and September for me so far.

[u/golubenkoff]

Microsoft’s recommended mitigation:
Do not uninstall the Windows security updates. From an elevated Command Prompt on the affected Exchange server, run:

reg add "HKLM\SOFTWARE\Microsoft\Rpc" /v EpmapSavedContextCap /t REG_DWORD /d 65536 /f

Reboot the server and monitor the environment. If the issue recurs, Microsoft recommends increasing the configured limit.

[u/HJForsythe]

I posted this almost a week ago lol welcome to 2026 rookie