How to Check Every Mailbox a Microsoft 365 User Can Access

In this article, we'll explore how to view all the mailboxes a user has access to, utilizing PowerShell and CoreView.

Find All Accessible Mailboxes Using PowerShell

Before You Start: PowerShell Requirements and Permissions

• Use the latest Exchange Online PowerShell V3 (EXO V3) module.

• Ensure you have the required Exchange administrative permissions.

PowerShell Steps to List Mailboxes a User Can Access

You can use the PowerShell command to list all the mailboxes a user has access to in Microsoft 365. Here are the steps to do so:

1. Open Windows PowerShell as an administrator.

2. Connect to Exchange Online:

Connect-ExchangeOnline -UserPrincipalName <your_admin_UPN>

3. List mailboxes a user has full access to
   Replace <your_admin_UPN> with your administrator's User Principal Name.

$User = "<UPN>"
Get-Mailbox -ResultSize Unlimited | ForEach-Object {
   $mailbox = $_
   Get-MailboxPermission -Identity $mailbox.Identity |
      Where-Object { $_.User -eq $User -and $_.AccessRights -contains "FullAccess" }
} | Select-Object Identity, User, AccessRights

4. This script will return a list of all mailboxes where the specified user has Full Access permissions.

For other permission types (e.g., SendAs, Read), modify the script by adjusting or removing the -and $_.AccessRights -contains "FullAccess" part.

Identify User Mailbox Access with CoreView

You can use CoreView to see all the mailboxes a user has access to. Please follow the steps below to do so:

• Log in to the CoreView portal
• Under “Reports”, select “Security”, and then choose “User mailbox security”.

• Here, apply a filter to the “Users with access” column using the user principal name of the user for whom you want to view all mailbox access.
• This will display all the mailboxes to which the selected user has access rights.

The “Full name”, “User principal name”, and “Primary SMTP address” columns provide details about the mailboxes the user has access to. The “Access rights” column indicates the level of access the user has to each respective mailbox.

Alternatively, you can access this information from the user's “User card”.

• Search for the user using the search bar located in the top menu.
• Under “Mailbox”, click on “See details”.

• A wizard will appear. Under the “Permissions to other mailboxes" tab, you will see all the mailboxes to which the selected user has access rights.