Microsoft has released Security Updates (SUs) for vulnerabilities found in:
Exchange Server Subscription Edition (SE)
Exchange Server 2019
Exchange Server 2016
SUs are available for the f...
Updated Jun 15, 2026
Version 4.0
Blog Post
We installed the June update and are experiencing issues with Shared Mailboxes where 'MessageCopyForSentAsEnabled' is enabled.
We are still in Hybrid mode with Centralized Transport enabled with almost all of our users in Exchange Online.
After installing the June update users are still able to send mails from the shared Mailboxes, and a copy of the sent mail appear in the sent items folder in the shared mailbox.
The issue arrise when the mail is sent to an external recipient and routed through the on-premises servers. The on-premises hybrid servers uses the builtin transport agent 'SharedMailboxSentItemsRoutingAgent' and send a forwarded message back to the Shared Mailbox with the original mail attached.
Is this something you have seen and have a solution for?
Nino_Bilic
Microsoft
MicrosoftWe are investigating this; it is proving not very straightforward to reproduce (it does not seem to repro in every hybrid org with centralized mailflow enabled). We are making progress, though. It would help if folks who ran into this opened support tickets.
EDIT 2 (7/7/2026): The issue is now fully understood. We are working on steps that can provide relief for on-premises customers until we can address this in an update. I'll update the blog post once we have a way forward.
Thanks Nino, I appreciate the investigation and the honesty about the root cause.
To be blunt, though: from a customer's perspective it doesn't really matter which internal team owns the bug, or whether it originates on-premises or service-side, or something else. As long as Microsoft supports and allows Centralized Mail Transport in an Exchange Hybrid setup, this behavior breaks a supported configuration, and that makes it Microsoft's problem to fix, regardless of internal ownership.
For what it's worth, we raised a separate support case and were told this is a confirmed, known issue with a fix currently being deployed, plus a follow-up next week to verify it reached our tenant. So the fix does seem to be in motion. Given how many people are tracking this exact thread, it would help everyone if that status were reflected here too, even just a short "known issue, fix rolling out", rather than pointing us all to individual support tickets.
Also, why is there nothing to see in the Service Health Portal, like Krusty_the_admin is asking...?In the meantime, the workaround we were given is to add the shared mailbox in Outlook as a separate account, so sent items are stored in the shared mailbox itself.
Is it reported at M365 Service Health...? :-/
Yes, same issue appearing only after the June SU... We did raise a support case as well and hope you can find the issue.
When one affected user sends or forwards a message in the name of a shared mailbox, the user receives an unexpected confirmation/forwarded message afterwards. The message contains the original mail as an Outlook item attachment and includes the following text:
The attached message was sent by a member of this shared mailbox. Usually, it would appear in Sent Items, but your service is currently being upgraded. Things will be back to normal after the upgrade.
Environment:
- Exchange Hybrid
- Exchange Online shared mailbox
MessageCopyForSentAsEnabled : True
MessageCopyForSendOnBehalfEnabled : True
- Centralized mail routing / centralized transport enabled
- External mail from Exchange Online is routed through the on-premises Exchange transport path
- Exchange Server SE RTM Jun26SU installed (KB5094139 installed)
- Nino_Bilic
As I already stated, this was caused by a service change. It is my understanding that a change that will make this go away is being deployed in the service already (rolling out).
hard to find in the comment box. Is it listed under known issues?
So then the issue is triggered by the installation of then June SU.
Hi Nino
We opened a support ticket wednesday - feel free to reach out if you need a case number or any other information.
