Tenant Migration

How to Add an Old Microsoft 365 Domain as a User Alias in Bulk

Rana Garcia Oct 16, 2026 869 views

Question details

Administrators need to add an old domain as an email alias for multiple Microsoft 365 users to ensure uninterrupted mail delivery after a tenant domain change.

How to Add an Old Microsoft 365 Domain as a User Alias in Bulk

Product: Microsoft 365
Device & OS: not provided
Scenario: Changing domains within the same Microsoft 365 tenant while preserving old email addresses.
Observed behavior: Users need to continuously receive messages sent to their old domain address at their new primary email address.

Before you start

Ensure you have Exchange Administrator privileges in Microsoft 365, and verify that the old domain is still actively registered and verified within your tenant's domain settings.

Solution 1Recommended

Automate Bulk Alias Assignment using PowerShell

Use Exchange Online PowerShell to automate adding the old domain alias to multiple users simultaneously.

Because the Microsoft 365 Admin Center lacks a bulk update feature for email aliases, using PowerShell is the most efficient way to assign an old domain alias to multiple users across your tenant.

Connect to Exchange Online

Open PowerShell as an administrator and run the 'Connect-ExchangeOnline' command to authenticate with your Microsoft 365 tenant.

Prepare your user list

Create a CSV file containing the user principal names (UPNs) and the old domain email addresses you wish to add as aliases.

Run the bulk update script

Use a 'foreach' loop with the 'Set-Mailbox' cmdlet to iterate through your CSV and append the old domain address to each user's EmailAddresses property.

Automate Bulk Alias Assignment using PowerShell

Testing Delivery: Always verify the domain status and send a test email to the old alias address before finalizing your migration.

Solution 2

Add an Alias Manually via Microsoft 365 Admin Center

Best for adding an old domain alias to a small number of users individually.

Free Microsoft Office alternative

Looking for a Lightweight and Cost-Effective Office Suite?

While managing Microsoft 365 tenants and domain migrations, you might be seeking ways to optimize software licensing costs. WPS Office offers a free, highly compatible alternative to Microsoft Office for your users.

1. Download the installer: Visit the official WPS Office website and download the free installation package for your operating system.
2. Install and launch: Run the installer and follow the on-screen instructions to set up WPS Office on your device.
3. Open your files: Launch WPS Office to instantly open and edit your existing Microsoft Office documents without any formatting loss.

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microsoft office alternative - wps office

Frequently Asked Questions

Can I add domain aliases in bulk using the Microsoft 365 Admin Center interface?

No, the Microsoft 365 Admin Center interface only allows you to add email aliases to users one by one. Bulk operations for aliases require using PowerShell or Microsoft Graph.

Do users need an extra license for an email alias?

No, adding an email alias to an existing Microsoft 365 user account is completely free and does not require an additional license.

How many aliases can I add to a single Microsoft 365 user?

You can add up to 400 email aliases per user mailbox in Microsoft 365.

Can a user log in with their old domain alias?

No, users must always log in using their primary email address (User Principal Name). However, they will successfully receive any emails sent to their associated aliases.

Migration & Recovery