Shared Mailbox is not shown in web Outlook when trying to add it

--SAL-- 0 Reputation points
2026-10-05T09:42:11.76+00:00

Hello,

a client of mine needs the exakt date of mails a former colleague sent over five years ago. The former colleagues mailbox still exists as a shared mailbox. So I gave my client access to that mailbox.

In Outlook (classic) they can see the shared mailbox. Because the mails are older than five years they have to set the "Exchange-Cache-Mode" to "all" to see the wanted mails. The result was that Outlook was so slow, that it didn't properly work.

The obvious solution was to access their mailbox with the web version of Outlook. Untfortunatly they can't add the shared mailbox, nor can they open the mailbox in another tab, when clicking on their profile picture in the top right corner.

Other shared mailboxes are shown, but not the one in question. It simply does not appear in the search results.

I already tried to set:

Set-Mailbox

But that didn't work.

The client works with an Exchange Online server.

Thanks in advance fpr your help.

Outlook | Web | Outlook on the web for business | Email
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Answer accepted by question author
Demi Nguyen 2,330 Reputation points Independent Advisor
2026-10-05T10:23:54.5866667+00:00

Hello --SAL--,

Thank you for the detailed explanation. Since the shared mailbox opens correctly in Classic Outlook but cannot be found in Outlook on the web, I recommend checking its delegation settings in the Exchange admin center.

Please sign in to the Exchange admin center, then go to:

Recipients > Mailboxes > select the affected shared mailbox > Delegation

Under Read and manage (Full Access), confirm that the affected user is listed. Full Access allows the user to open the shared mailbox and view or manage its contents.

User's image

If Full Access has already been assigned directly for some time but Outlook on the web still cannot find the mailbox, you can try removing and reassigning the permission:

  • Remove the affected user from Read and manage (Full Access).
  • Save the change and allow some time for it to process.
  • Add the user again directly under Read and manage (Full Access).
  • Save the change and wait for the updated permission to synchronize.
  • Ask the user to sign out of Outlook on the web.
  • Open a new InPrivate or Incognito window and try again through "Open another mailbox"

User's image

Please note that Send As permission is only required if the user needs to send messages as the shared mailbox. For opening and reviewing the mailbox contents, Full Access is the relevant permission.

For additional information, please refer to the official Microsoft documentation: Shared mailboxes in Exchange Online | Microsoft Learn

Please update me with the permission status and the result after removing and reassigning Full Access.

Kind regards,

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