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.”
Workaround
To work around this issue, create a setting override by running the following command in an elevated Exchange Management Shell window. This disables the change that was introduced in the June 2026 Security Update (SU) for Exchange Server.
1. New-SettingOverride -Name "DisableBlockSharedAndUserMailboxHeaders" -Component Transport -Section BlockSharedAndUserMailboxHeaders -Parameters @("Enabled=false") -Reason "Mitigate Sent Items issue in Exchange hybrid"
2. Get-ExchangeDiagnosticInfo -Process Microsoft.Exchange.Directory.TopologyService -Component VariantConfiguration -Argument Refresh
More information
Microsoft is investigating this issue and will update this article as more information becomes available.