Convert User Mailbox to Shared Mailbox in Exchange Server or Microsoft 365

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The shared mailbox in Exchange is similar to a regular user mailbox. The main difference is that a shared mailbox can be used by more than one user at the same time. Users access a shared mailbox by using their own credentials after they have been granted the appropriate permissions, and the shared mailbox doesn’t require a separate license (Client Access License or subscription). In this article, we’ll look at how to convert a user mailbox to a shared mailbox in on-premises Exchange Server and in Exchange Online (Microsoft 365).

Shared mailboxes overview

Shared mailboxes are typically used by members of a department or team to access a common pool of emails. Shared mailboxes are also often used when an employee leaves the company, but others still need access to the contents of that mailbox. In this case, the user account can be disabled for security reasons, and the mailbox can be converted to a shared mailbox.

Convert a User Mailbox to a Shared Mailbox in Exchange Server

You cannot use Exchange Admin Center (EAC) to convert a user mailbox to a shared mailbox in on-premises Exchange Server 2019/2016. This conversion can only be performed by using PowerShell from the Exchange Management Shell (EMS).

Open the EMS console and check the mailbox type by running the following command:

Get-Mailbox -Identity v.wolf@theitbros.com | Format-Table Name, RecipientTypeDetails

We found that the mailbox type is UserMailbox. Let’s convert it into a shared mailbox:

Set-Mailbox v.wolf@theitbros.com -Type Shared

Check that the mailbox type has changed to SharedMailbox.

Now check if the mailbox type has changed to SharedMailbox:

Get-Mailbox -Identity v.wolf@theitbros.com |
Format-Table Name, RecipientTypeDetails

After converting the mailbox, it’s common practice to disable the associated AD account if the user no longer requires interactive sign-in. Open the ADUC console (dsa.msc) and make sure that the user account is disabled and cannot be used to log on to the domain.

To allow other users to access the shared mailbox and view its contents, you need to assign them the appropriate permissions.

Let’s grant the user v.ebner the Full Access and Send As permissions to a shared mailbox:

Add-MailboxPermission -Identity v.wolf -User resource\v.ebner -AccessRights "FullAccess"

Add-ADPermission -Identity v.wolf -User v.ebner -ExtendedRights "Send As"

Note. In on-premises Exchange Server, Send As permissions are assigned by using the Add-ADPermission cmdlet because the permission is stored in AD. In Exchange Online (Microsoft 365), the equivalent permission is granted by using the Add-RecipientPermission cmdlet, as shown later in this article.

Note that a shared mailbox supports Send As and Send on Behalf permissions. You can config Send on Behalf using PowerShell with the Set-Mailbox -GrantSendOnBehalfTo parameter:

Set-Mailbox -Identity v.wolf@theitbros.com -GrantSendOnBehalfTo v.ebner

This allows v.ebner to send messages on behalf of the shared mailbox. Recipients will see a sender such as v.ebner on behalf of v.wolf@theitbros.com. User v.ebner can now add the shared mailbox in Outlook, access its contents, and send messages using the shared mailbox address. With Send As, recipients see the shared mailbox address as the sender. With Send on Behalf, recipients see the individual user as the sender on behalf of the shared mailbox.

Open the user’s OWA, right-click the user name, and select Add Shared Folder.

Type the shared mailbox name you want to add.

The shared mailbox will appear in the user’s Outlook folder tree.

How to Convert a User Mailbox to a Shared Mailbox in Microsoft 365

You can convert a user’s mailbox to a shared mailbox in Exchange Online (Microsoft 365) from the Exchange Admin Center (EAC):

1. Login to https://admin.exchange.microsoft.com
2. Go to Recipients > Mailboxes
3. Click the user mailbox you want to convert and then navigate to the Others tab
4. Click Convert to shared mailbox

Convert EOL user mailbox to shared mailbox using PowerShell

Or you can convert an EOL user mailbox to a shared mailbox using PowerShell:

1. Connect to Microsoft tenant using the Exchange Online PowerShell module:
   

   Connect-ExchangeOnline

2. Convert a mailbox to a shared:
   

   Set-Mailbox tnc-salary -Type Shared

3. Check the mailbox type changed to SharedMailbox:
   

   Get-Mailbox -Identity tnc-salary | fl Name, RecipientTypeDetails

   
4. Now allow other users to access the shared mailbox:
   

   Add-RecipientPermission tnc-salary@theitbros.com -Trustee b.jackson -AccessRights SendAs 
   
   Add-MailboxPermission -Identity tnc-salary@theitbros.com -User b.jackson -AccessRights FullAccess -AutoMapping:$true -InheritanceType All

   Note that when -AutoMapping:$true is specified, Outlook automatically adds the shared mailbox to the user’s profile after Autodiscover refreshes. You should use -AutoMapping:$false if you want users to add the mailbox manually.

Note. Keep in mind that permission changes aren’t always applied immediately. It can take several minutes (or longer in Microsoft 365) before Outlook/Outlook on the web reflects the new access rights.

No separate Microsoft 365 license required for shared mailbox. Thus, you can create an unlimited number of shared mailboxes with a maximum size of 50 GB per mailbox.

Note. Shared mailboxes over 50GB require a separate Exchange Online Plan 2 license.

Considerations

• keep in mind that mailbox conversion doesn’t remove mailbox data
• Outlook may require time before AutoMapping occurs
• permission changes aren’t immediate
• mailbox type conversion doesn’t remove litigation hold

Bulk convert multiple mailboxes to shared mailboxes

In enterprise environments, converting mailboxes to shared mailboxes is often part of an employee offboarding process. Instead of converting each mailbox individually, you can automate the process by importing a list of users from a CSV file. For example, if your SharedMailboxes.csv file contains the following:

UserPrincipalName
user1@contoso.com
user2@contoso.com
user3@contoso.com

You can convert all listed mailboxes by running the following command:

Import-Csv .\SharedMailboxes.csv | ForEach-Object {
Set-Mailbox -Identity $_.UserPrincipalName -Type Shared
}

In production environments, you should consider adding error handling and logging to identify mailboxes that could not be converted successfully.

After the conversion, you can extend the script to automate additional offboarding tasks, such as:

• Disabling the associated user account (if required).
• Removing or reassigning the Microsoft 365 license.
• Granting Full Access and Send As permissions to other users.
• Sending notifications to managers or IT staff as part of the employee offboarding workflow.

Note. Before running bulk mailbox conversions, you should consider testing the script against a small set of mailboxes to verify the expected results and avoid unintended changes in your production environment.

Wrapping Up

Converting a user mailbox to a shared mailbox is a common admin task in both Exchange Server and Exchange Online. PowerShell provides the most flexible approach for both single mailbox conversions and bulk offboarding cases, while the Exchange Admin Center offers a simple graphical option for Microsoft 365 admins.