Identify Problematic Shared Mailboxes using Sign-in Logs

Exchange Online shared mailboxes have Entra ID accounts. The accounts have passwords and people can sign-into the account and start a mail client that’s connected to the shared mailbox to process email. Is this a problem? Absolutely!

Shared mailboxes don’t require Exchange Online or any other licenses unless the mailboxes have an archive, need more than 50 GB quota, use litigation hold, or are subject to Purview retention policies. As stated in the Microsoft service description:

“To access a shared mailbox, a user must have an Exchange Online license, but the shared mailbox doesn’t require a separate license.”

No Need Exists to Sign Into Shared Mailboxes

Shared mailboxes are intended for joint access by multiple users whose connections are controlled by permissions managed by Exchange Online. Full Access permission allows a user full control over all mailbox folders and items while Send As or Send on Behalf Of allows them to send email from the mailbox. No need exists to sign into the Entra ID accounts for shared mailboxes, and if you sign into an unlicensed shared mailbox, you violate Microsoft licensing terms.

One reason I have heard advanced to justify signing into a shared mailbox is after someone leaves the organization and their mailbox is converted to a shared mailbox. If the mailbox includes some information that’s important to the organization, another user might need to sign into the mailbox to retrieve the data. I don’t buy this logic. Granting Full Access permission to the mailbox is sufficient to review the items stored there. I prefer to use inactive mailboxes to preserve ex-employee content instead. It’s just a cleaner solution.

Microsoft documentation says:

“A shared mailbox is a type of user mailbox that doesn’t have its own username and password. As a result, users can’t log into them directly.”

This is factually incorrect. Every shared mailbox has an ExternalDirectoryObjectId property that points to its Entra ID account. This PowerShell snippet uses the property to report the user principal names for the accounts:

$Mbx = Get-ExoMailbox -RecipientTypeDetails SharedMailbox -ResultSize Unlimited | Sort-Object DisplayName
ForEach ($M in $Mbx) {
    $User = Get-MgUser -UserId $M.ExternalDirectoryObjectId
    Write-Output ("Mailbox {0} has Entra ID account {1}" -f $M.DisplayName, $User.UserPrincipalName)
}
Mailbox Admin-RA-Shared has Entra ID account admin-ra-shared@office365itpros.com
Mailbox Azure Management Account has Entra ID account Azure.Management.Account@office365itpros.com

Changing the password and enabling the accounts to allow users to sign into the accounts is easy. If you don’t want to use PowerShell, you can select the account in the Microsoft 365 admin center and perform the actions there (Figure 1).

Checking for Illegal Shared Mailboxes

Life isn’t perfect and people make mistakes. It’s possible that a tenant has some shared mailboxes that fall in a technically illegal state because people sign into the mailbox instead of connecting using mailbox permissions. To detect these situations, we can use the Get-MgAuditLogSignIn cmdlet to check if any sign-in records exist for the mailbox accounts. The account running the script must have an Entra ID P1 license to access the audit log records.

To illustrate the point, I wrote a script (downloadable from GitHub) to find shared mailboxes and check if they’ve been signed into. If so, a further check establishes if the mailbox’s account is licensed with Exchange Online Plan 1 or Plan 2. The output is shown in Figure 2.

Fortunately, the two mailboxes with detected sign-in records both have Exchange Online Plan 2 licenses, so they’re in compliance.

Other Checks

Microsoft doesn’t check shared mailboxes where other license requirements arise, like those with archive mailboxes or those on litigation hold. If you want to scan for those conditions, the necessary code is covered in this article. It wouldn’t take much to combine the two scripts to have one script that checks everything. I’ll leave that as an exercise for the reader.

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