Outlook (Classic) - Cannot Send New Emails from a Shared Mailbox

Question

Outlook (Classic) - Cannot Send New Emails from a Shared Mailbox

Nadir Hasan 0

Hi everyone,

I'm experiencing an issue with a mapped shared mailbox that appears to be specific to Outlook (Classic).

The affected user can successfully:

Send, receive, forward from the shared mailbox using Outlook on the Web (OWA)
Send, receive, forward emails from the shared mailbox using Outlook for Windows
Receive, forward emails but cannot send new emails in Outlook (Classic)

However, when attempting to send a new email from the shared mailbox in Outlook (Classic), the send operation fails. This behaviour has been confirmed on multiple devices, which suggests the issue may not be device-specific. The user reports that sending new emails from the shared mailbox was working until approximately five days ago. At that point, they began receiving an NDR indicating that they did not have the required permissions to send from the mailbox.

Following troubleshooting, the NDR is no longer generated. Instead, Outlook (Classic) now displays the following error message:

"You do not have permission to send the message on behalf of the specified user."

To rule out an issue with Outlook selecting a cached or previously used sender address, the user has confirmed that when composing a new message they manually type and select the shared mailbox address in the From field rather than choosing an address from the autocomplete or recently used address list.
What makes this particularly confusing is that the user can still successfully send new emails from the same shared mailbox using OWA and New Outlook.

Has anyone encountered this behaviour before? Are there any known issues or additional troubleshooting steps that would explain why:

Sending works in OWA and New Outlook
Receiving and forwarding work in Outlook (Classic)
Sending new emails fails only in Outlook (Classic) with a permissions-related error

Any guidance would be greatly appreciated.

Thank you.

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1 answer

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Answer 1

Hi @Nadir Hasan

Thank you for the details. I’m a fellow community user, so I can’t check your tenant, but here is my thought.

Since the user can already send from the shared mailbox in OWA and New Outlook, the permission itself seems to be working on the server side. Outlook (Classic) can take longer to pick up a newly granted or changed permission, because it relies on settings synced to the local profile.

Could you please try the following?

1.Wait about one day if the Send As or Send on Behalf permission was granted or changed recently, then try sending a new email from Classic again.

2.Because the issue started recently, an admin can try removing and re-granting the relevant send permission. Please also verify the existing permissions with Exchange Online PowerShell, rather than relying only on what the Exchange admin center shows.

3.In Classic, temporarily test the account in Online Mode. If sending works in Online Mode but not in Cached Exchange Mode, the problem may be related to delegation data stored in the OST or Outlook’s cache. This option may be grayed out. If so please contact your admin to change the policy that is locking it
4.Remove the shared mailbox from the Classic profile and add it again. Automapping is on by default, so the shared mailbox appears in Classic automatically while the user has Full Access, and the user can’t remove it from Outlook themselves.

To remove it, an admin can remove the user’s Full Access permission in the Exchange admin center (Manage mailbox delegation > Read and manage). The mailbox should disappear from the profile after some time. Please keep the Send As/Send on Behalf permission unchanged. To add it back without automapping, an admin runs the following in Exchange Online PowerShell. The first command removes Full Access, and the second grants it again with automapping disabled:

Remove-MailboxPermission -Identity <SharedMailbox> -User <User> -AccessRights FullAccess -Confirm:$false
Add-MailboxPermission -Identity <SharedMailbox> -User <User> -AccessRights FullAccess -AutoMapping $false

Then the user adds the mailbox manually in Classic: File > Account Settings > Account Settings > select the Exchange account > Change > More Settings > Advanced > Add. Type the shared mailbox name, click OK, then Next and Finish. Menu names can differ slightly between versions.
Please refer the following article: Remove automapping for a shared mailbox | Microsoft Learn

5.Create a brand-new Outlook profile: Control Panel > Mail > Show Profiles > Add. Configure only the user’s primary mailbox, then test the shared mailbox again. The issue occurring on multiple devices doesn’t fully rule out a profile problem if the same account and the same automapped configuration are used on all of them.

Please let me know the result.