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Bulk Convert Exchange Online User Mailboxes to Shared Mailboxes

Maira Mehtab Sep 22, 2026 869 views

Question details

An administrator needs to automate the conversion of approximately 950 licensed Exchange Online user mailboxes to shared mailboxes, set up forwarding rules, and remove licenses following a company merger.

Product: Exchange Online
Device & OS: not provided
Scenario: Post-merger IT consolidation requiring bulk modification of mailbox types, mail routing configurations, and license management for a large group of users.
Observed behavior: The goal state is to have all 950 user mailboxes successfully converted to shared mailboxes, configured with specific forwarding rules, or converted to mail users, with their Microsoft 365 licenses safely removed to optimize costs.

Before you start

Ensure you have Global Administrator or Exchange Administrator privileges in your Microsoft 365 tenant, and prepare a CSV file containing the target user identities. Always test your conversion script on a small group of 2-3 mailboxes before executing the bulk change on all 950 users.

Solution 1Recommended

Use Exchange Online PowerShell for Bulk Conversion

The most efficient and reliable method to convert hundreds of mailboxes simultaneously is by using Exchange Online PowerShell cmdlets.

By utilizing PowerShell in combination with a CSV file, administrators can automate the conversion process, apply forwarding addresses, and handle external mail users without manually clicking through the Microsoft 365 Admin Center for every user.

Connect to Exchange Online

Open PowerShell as an administrator and run the 'Connect-ExchangeOnline' cmdlet. Sign in with your Global Administrator credentials.

Convert Mailboxes to Shared

Use a script to loop through your CSV file and run the 'Set-Mailbox -Identity "user@domain.com" -Type Shared' command for each targeted user mailbox.

Configure Email Forwarding

If incoming emails need to be routed elsewhere, apply the 'Set-Mailbox -Identity "user@domain.com" -ForwardingSmtpAddress "target@domain.com" -DeliverToMailboxAndForward $true' command.

Create External Mail Users (Optional)

If an external mail user is required rather than a shared mailbox, use the 'New-MailUser' cmdlet to create the necessary objects.

Remove Microsoft 365 Licenses

Once the mailboxes are successfully converted and retention policies are verified, use Microsoft Graph PowerShell or your approved licensing commands to remove the Exchange Online licenses from the users.

Compliance and Access Review: Before removing any licenses, carefully review your organization's data retention, forwarding, compliance, and access requirements to ensure no critical data is lost during the license removal phase.

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Frequently Asked Questions

Do I need to assign a license to a shared mailbox in Exchange Online?

Generally, shared mailboxes do not require a separate license unless they exceed 50 GB in storage size, require an in-place archive, or have a litigation hold applied. Always verify the mailbox size before removing the original user license.

Can I bulk convert mailboxes using the Microsoft 365 Admin Center UI?

The Microsoft 365 Admin Center is designed for single mailbox conversions. For bulk operations involving hundreds of users, Exchange Online PowerShell is the only practical and supported method.

What happens to the user's OneDrive data when I remove their Exchange license?

Removing a user's Microsoft 365 license will flag their associated OneDrive data for deletion based on your organization's retention policies (typically 30 days). Ensure you back up or transfer necessary files before stripping licenses.

How do I safely test my PowerShell bulk conversion script?

Create a test CSV file containing only 2 or 3 test user accounts. Run your PowerShell script against this limited list, then manually verify the mailbox types, forwarding rules, and access permissions before processing the full batch.

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