Troubleshooting Issue with Sending Emails from a Shared Mailbox in Microsoft 365

Have you ever encountered a situation where a delegate with full permissions and SendAs rights on a shared mailbox still couldn’t send emails, receiving bounce-backs stating, “You do not have permission to send the message on behalf of the specified user”? If so, you’re not alone. This issue often stems from a hidden setting that’s not immediately apparent in the Exchange Admin Center (EAC).

Understanding the Problem

In Microsoft 365 (formerly Office 365), managing permissions and access to shared mailboxes is critical for effective collaboration. Users granted SendAs permissions should theoretically be able to send emails on behalf of the shared mailbox without issues. However, sometimes despite setting these permissions correctly via PowerShell commands like Set-Mailbox, the problem persists.

Set-Mailbox -Identity -GrantSendOnBehalfTo

The Hidden Attribute Issue

One less-known issue that can cause this problem is when the shared mailbox is hidden from the Global Address List (GAL). This attribute, if enabled or set to true, hides the shared mailbox’s email address from the address book. As a result, even though the delegate has permissions to send emails on behalf of the mailbox, the address isn’t visible for selection in the Outlook address book or when manually entering the recipient’s email.

Resolving the Issue

To resolve this issue, follow these steps:

  1. Check and Update GAL Visibility:
    • Verify Hidden Attribute: Use Exchange Online PowerShell to check if the shared mailbox’s HiddenFromAddressListsEnabled attribute is set to true.

Get-Mailbox | Select HiddenFromAddressListsEnabled

  1. Disable Hidden Attribute: If it’s set to true, disable it to ensure the shared mailbox’s email address appears in the GAL.

Set-Mailbox -Identity -HiddenFromAddressListsEnabled $false

Update Address Book:

  • Force Address Book Update: After disabling the hidden attribute, force the address book update to ensure changes reflect immediately.
    • In Outlook: Go to File > Account Settings > Download Address Book and click OK to update.
    • In Exchange Online PowerShell:

Update-OfflineAddressBook “Default Offline Address Book”

Test Sending Emails:

  • After completing the above steps, the shared mailbox’s email address should be visible in the address book.
  • Delegate users with SendAs permissions should now be able to send emails on behalf of the shared mailbox without encountering permission issues.


Managing shared mailbox permissions and addressing unexpected issues like hidden attributes in Microsoft 365 can sometimes be complex. By understanding and addressing issues such as GAL visibility settings, you can ensure smooth collaboration and effective email management within your organization.

If you’re facing similar challenges or have encountered other nuances in managing Microsoft 365 environments, feel free to share your experiences or ask questions in the comments below.



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