New Outlook doesn't allow me to sign in to shared mailbox

Question

In the classic version of Outlook I was loged into my personal mailbox and I also added a shared mailbox that we use together with colleagues.

After switching to the New Outlook I got a prompt that I should log in to the shared mailbox.

After logging in I got a message that Account is already added and the only thing I can do is removing it. I removed it and added it again, but the same error occured.

However, when switching back to the old Outlook everything works normally. Is something wrong with shared mailbox persmissions or my client settings?

Answer 1

There are currently many problems with the new Outlook, and this one has blocked many users already. Below are some user complaints and answers by Microsoft personnel, as well as one workaround suggested by a user.

Post Shared Mailbox does not work on new Outlook look from January 17, 2024, answer by Inema Elyse from MSFT:

After thorough research and testing I found that you are right you are now prompted for a password when trying to add a shared or user mailbox in new Outlook. Kindly understand that The New Outlook is in a Preview state, and they are still working on the platform/foundation of the application itself. At this stage of the development, luckily, restarting the client is very quick. As an alternative, you can switch back to "Classic" Outlook by turning New Outlook Off

Post Unable to add Shared mailbox on New Outlook from January 10, 2024, answer by Simbarashe Gungano from MSFT:

However, if you cannot see the shared mailbox still, kindly revert to classic Outlook and close your Outlook and start over the process again. Make sure you migrate your settings from classic Outlook to new as you switch to new Outlook. Migrate your settings to the new Outlook for Windows - Microsoft Support.

If the issue persists, please reach out to new Outlook for Windows support team, as new Outlook is still in preview/production

As you can see, Microsoft has no good solution to the problem, but a workaround was proposed by user Patrick Bergstrom in post Unable to add shared mailbox in New outlook for Mac - Workaround Found:

I realized, the only shared inbox I was able to add to New Outlook, was one I had sent a test email to, to verify it was working. This caused it to show up in the list of available inboxes. Strangely, I had also sent an email to info@chemex, which when I searched for an "info" shared inbox I own, that showed up in the list.

This led me to sending a test message to all my shared inboxes. As a result, I was able to add them all in the new Outlook for Mac.

It seems like the "Shared Inbox" function is not correctly connecting to the M365 service to populate the list of available inboxes in New Outlook, and it doesn't have the same option to force it that old outlook and outlook.com do. Add back the "use " option when no search results are found, and it should fix this.

Try this workaround, but if it doesn't work for you, then I suggest to revert to the classic Outlook for at least some months, in the hope that this will be fixed in the future.

Answer 2

I contacted MS Support that provided me with the solution.

If you have access to the shared mailbox in OWA, and if you are trying to add the shared mailbox in New Outlook, right-click the name of the primary mailbox, and then choose "Add a shared folder or mailbox." Enter the shared mailbox account. The mailbox will be visible in the “Shared with me” section of the account.

After that you find the shared mailbox visible in the tree:

Answer 3

I just found that the shared mailbox was hidden from the Global Address List (GAL). I unchecked "hide", reset my outlook, and was able to add the shared mailbox. Afterwards I hid the mailbox again. All is well now.