Office 365 Mailbox Access: Grant User Permissions & Delegate Mailbox

This guide covers how to grant full mailbox access using both the Microsoft 365 Admin Center and PowerShell.

Requirements

• Admin access to the Microsoft 365 Admin Center or Exchange Online PowerShell.
• The target mailbox must be an active account.

Method 1: Grant Full Access Using the Microsoft 365 Admin Center

Step 1: Log into the Admin Center

1. Go to https://admin.microsoft.com.
2. Sign in with your administrator credentials.

Step 2: Open the User’s Mailbox Settings

1. In the left-hand menu, click Users > Active users.
2. Find and select the user whose mailbox you want to grant access to.
3. Under the Mail tab, click Mailbox permissions.

Step 3: Assign Full Access Permissions

1. Under Mailbox permissions, locate Read and manage (Full Access).
2. Click Edit and then Add permissions.
3. Search for and select the user who should have full access to the mailbox.
4. Click Save changes to apply the permissions.

📌 Note: It may take up to 60 minutes for the changes to take effect.

Method 2: Grant Full Access Using PowerShell

For larger organizations, PowerShell provides a faster way to grant mailbox access permissions in bulk.

Step 1: Connect to Exchange Online PowerShell

1. Open PowerShell on your computer.
2. Run the following command to connect to Exchange Online:powershellCopyEditConnect-ExchangeOnline -UserPrincipalName admin@yourdomain.com

Step 2: Grant Full Access to a User

Run the following command, replacing values as needed:

Add-MailboxPermission -Identity "targetuser@yourdomain.com" -User "granteduser@yourdomain.com" -AccessRights FullAccess

📌 Example: If you want to give John Smith full access to Jane Doe’s mailbox, use:

Add-MailboxPermission -Identity "jane.doe@yourdomain.com" -User "john.smith@yourdomain.com" -AccessRights FullAccess

Step 3: Verify the Permission Assignment

To confirm that full access was granted, run:

Get-MailboxPermission -Identity "jane.doe@yourdomain.com"

This will display a list of users who have access to the mailbox.

How the Assigned User Can Access the Mailbox

Outlook on the Web (OWA):

1. Log in to https://outlook.office.com.
2. Click on the profile picture (top-right corner).
3. Select Open another mailbox and enter the mailbox email address.
4. Click Open to access the mailbox.

Outlook Desktop App:

1. Open Outlook and go to File > Account Settings > Account Settings.
2. Select your account and click Change > More Settings.
3. Navigate to the Advanced tab and click Add.
4. Enter the shared mailbox email address and click OK.
5. Restart Outlook, and the mailbox will appear in the left-hand panel.

How to Remove Full Access Permissions

Using Admin Center:

1. Go to Admin Center > Users > Active Users.
2. Select the original mailbox owner.
3. Under Mailbox permissions, find the assigned user.
4. Click Remove and Save changes.

Using PowerShell:

Run the following command to revoke access:

Remove-MailboxPermission -Identity "jane.doe@yourdomain.com" -User "john.smith@yourdomain.com" -AccessRights FullAccess

Best Practices

• ✅ Review Mailbox Access Regularly – Ensure only authorized users have access to sensitive mailboxes.
• ✅ Use Role-Based Access Control – Instead of granting full access, consider using Send As or Send on Behalf for security.
• ✅ Document Changes – Keep a record of mailbox access modifications for compliance and auditing.