Convert User Mailbox to Shared Mailbox and Vice Versa in Microsoft 365

You can use multiple mailbox types in Microsoft 365 and Exchange Online hybrid deployments:

• Regular user mailbox — the mailbox is associated with a specific Azure user;
• Resource mailbox — a special type of mailbox used to schedule resources (meeting rooms or equipment);
• Shared mailbox — is not assigned to a specific user account and can be used by multiple Microsoft 365 users at the same time.

The main difference between a shared mailbox and a user mailbox is that a shared mailbox is associated with a user account in Azure Active Directory, which cannot be used to sign in.

A shared mailbox does not require a separate Microsoft 365/Azure license. This means that you can create as many shared mailboxes as you need, each with a maximum size of 50 GB.

Note. If the shared mailbox is larger than 50GB, you must assign an Exchange Online Plan 2 license to the shared mailbox.

Exchange Online allows you to convert a user’s mailbox to a shared mailbox and vice versa. In this case, all of the e-mail messages in the mailbox will be left untouched. Only the mailbox type is changed.

You can use the Exchange Admin Center to convert a regular user’s mailbox to a shared mailbox from:

1. Sign-in https://admin.exchange.microsoft.com/#/
2. Navigate to Recipients > Mailboxes;
3. The list shows all types of mailboxes in EOL (Recipient type: SharedMailbox or UserMailbox);
4. Select the user mailbox you want to convert and go to the Others tab;
5. Click the Convert to shared mailbox button and confirm the conversion;
   

You can use PowerShell to convert a regular user’s mailbox to a shared mailbox. Connect to your EOL tenant using the Exchange Online PowerShell module:

Connect-ExchangeOnline

To convert the personal mailbox of the infodept user to a shared mailbox, execute the following command:

Set-Mailbox infodept -Type Shared

To check if the type of mailbox has been changed, run the command:

Get-Mailbox -Identity infodept | Format-List Name, RecipientTypeDetails

The RecipientTypeDetails parameter should be changed to SharedMailbox.

Once you have created a shared mailbox, you need to assign permissions to the users who can use it. For example, in order to grant full permissions to the b.jackson user on the infodept shared mailbox:

Add-RecipientPermission infodept@theitbros.com -Trustee b.jackson -AccessRights SendAs

Add-MailboxPermission -Identity infodept@theitbros.com -User b.jackson -AccessRights FullAccess -AutoMapping:$true -InheritanceType All

Note that a Microsoft 365 user needs a Microsoft Exchange Online subscription to access the public mailbox.

In the same way, you can convert a shared mailbox to a user mailbox.

• In the Exchange Admin Center: click the Convert to regular mailbox button in the mailbox properties
• In PowerShell:

  Set-Mailbox salary –Type Regular

This will change the mailbox type to UserMailbox.