Symptoms
Consider the following scenario:
Your organization uses an Exchange hybrid deployment.
A shared mailbox is hosted in Exchange Online.
A mailbox is hosted on an on-premises server in Microsoft Exchange Server.
The mailbox has Send As or Send on Behalf permissions for the shared mailbox.
The MessageCopyForSentAsEnabled or MessageCopyForSendOnBehalfEnabled parameter is enabled to save a copy of messages that are sent as, or sent on behalf of, the shared mailbox.
After you install the June 2026 Security Update (SU) for Exchange Server, messages that are Sent As, or Sent on Behalf of, the shared mailbox may also be delivered to the Inbox folder of the shared mailbox as attachments with the following wrapper message:
“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.”
Resolution
To fix this issue, install the following security update or later updates for your version of Microsoft Exchange Server:
- Description of the security update for Microsoft Exchange Server Subscription Edition RTM: September 8, 2026 (KB5121608)
- Description of the security update for Microsoft Exchange Server 2019 CU15: September 8, 2026 (KB5121609)
- Description of the security update for Microsoft Exchange Server 2019 CU14: September 8, 2026 (KB5121610)
- Description of the security update for Microsoft Exchange Server 2016 CU23: September 8, 2026 (KB5121611)
More information
Run the following command to check whether the override was previously created as a workaround for the issue:
Get-SettingOverride "DisableBlockSharedAndUserMailboxHeaders"If you receive the following error, no further action is required:
"The operation couldn't be performed because object 'DisableBlockSharedAndUserMailboxHeaders' couldn't be found".
If no error is returned and the override is present, run the following command to remove it:
Remove-SettingOverride "DisableBlockSharedAndUserMailboxHeaders"