Exchange - Shared Mailbox Access (not Mac mail)

As of Q4 2023 Microsoft have retired basic (password) authentication on shared mailboxes.

Shared mailboxes no longer have passwords.

Shared mailboxes no longer require licenses to be assigned in any circumstance.

To access a shared mailbox Exchange online users MUST complete a Modern (OAuth2) authentication flow.

Shared mailbox access is granted by way of an access token attached to the OAuth session.

To sign in directly to a shared mailbox.

1. Specify the shared mailbox email address as the target of the initial sign in.

2. When the OAuth sign in form is displayed, select Sign in with a different account.

3. Specify the User Principal Name and password of a user with permission to access the shared mailbox.

4. Complete any further MFA flow.

As of 11/22 Microsoft implemented various 'security improvements' that change the way shared mailboxes are accessed.

A. Recommended, unlicensed shared mailbox

Create the shared mailbox in the O365 Admin Portal

Add users to the mailbox

Set copy to sent items options

Outlook PC: The shared mailbox will be added to the Outlook mailbox and folder navigation tree automatically by the automap feature of Exchange

Outlook Mac: The shared mailbox must be manually added to the profile.  Open other mailbox in Outlook preferences > Accounts > Advanced > Delegates > Open other mailboxes

B. Windows PCs using Outlook app to access the shared mailbox directly

The shared mailbox account must be assigned an O365 license.  It is not necessary to set a password on the shared mailbox.  A password is assigned silently by the system during shared mailbox creation.

In the Control Panel Mail/Outlook applet enable the option to, Prompt for a profile to be used.  Do not attempt to create the profile from Control Panel.

Start the Outlook desktop app and create a new profile from the select profile dialogue.  Options > New

Outlook modern auth will prompt for an e-mail address.  Enter the e-mail address of the shared mailbox and click Connect.

Select Microsoft 365 as the mail system type.

Outlook modern auth will prompt for a password.  Click the Sign in with another account link.  Sign in as the user, NOT THE SHARED MAILBOX.  Click the Sign in to this app only link.  Entering the shared mailbox credentials into the sign in dialogue results in a failed login error.

C. Outlook on the Web (OWA) access to the shared mailbox directly

The shared mailbox account must be assigned an O365 license.  The password should be assigned manually.

In a web browser navigate to outlook.com.

Enter the e-mail address and password for the shared mailbox account.

Open the shared mailbox as the only or primary mailbox in the Outlook desktop app.