Email Migration

How to Fix an External SMTP Proxy Address Conflict in Microsoft 365

Phi Hung Vo Oct 9, 2026 869 views

Question details

The user needs to identify and resolve an issue where an external SMTP proxy address is already assigned to a different recipient during an email migration.

How to Fix an External SMTP Proxy Address Conflict in Microsoft 365

Product: Microsoft 365 / Exchange Online
Device & OS: not provided
Scenario: Migrating mailboxes or users to Microsoft 365 and encountering a duplicate email address error.
Observed behavior: The migration fails with an error stating that the external SMTP proxy address is already assigned to another recipient object.

Before you start

Ensure that you have administrative credentials for your Microsoft 365 tenant and that the Exchange Online PowerShell module is installed on your machine.

Solution 1Recommended

Identify and Resolve the Conflict Using Exchange Online PowerShell

This is the most reliable method to locate hidden objects or mailboxes holding the conflicting proxy address across your entire tenant.

In Microsoft 365, a proxy address can only belong to one recipient object at a time. Mailboxes created directly in the Microsoft 365 admin center might not immediately appear in the Exchange admin center (EAC), making PowerShell the best tool to find them.

Connect to Exchange Online

Open your PowerShell terminal as an administrator and connect to Exchange Online by authenticating with your Microsoft 365 admin credentials.

Search for the conflicting address

Run the following command to find the object holding the address: Get-EXORecipient -ResultSize unlimited | Where-Object {$_.EmailAddresses -match "<conflicting SMTP address>"} | fl Name, RecipientType, EmailAddresses.

Analyze the output

Review the returned details to see the Name and RecipientType of the object that currently owns the conflicting SMTP proxy address.

Remove or modify the duplicate address

Once identified, update the conflicting recipient object to remove or change the duplicate address, freeing it up for your migration.

Identify and Resolve the Conflict Using Exchange Online PowerShell

Verification: After removing the address, wait a few minutes for the changes to sync across the Microsoft 365 tenant before retrying the migration.

Solution 2

Manage Email Address Types via Exchange Admin Center (EAC)

If you already know which user or contact has the conflicting address, you can manually remove it through the graphical interface.

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Frequently Asked Questions

Why does an SMTP proxy address conflict error occur in Microsoft 365?

This error occurs because an SMTP proxy address must be completely unique within an Exchange Online tenant. If another mailbox, contact, or group already uses the address, Microsoft 365 will reject assigning it to a new object during migration.

Why can't I find the conflicting user in the Exchange admin center?

Some mailboxes or objects created directly in the Microsoft 365 admin center might not immediately appear as contacts or standard mailboxes in the Exchange admin center. Using the Get-EXORecipient PowerShell command ensures you search the entire directory.

What permissions do I need to run the Get-EXORecipient command?

You need to be assigned an administrative role, such as Exchange Administrator or Global Administrator, to connect to Exchange Online PowerShell and execute recipient query commands.

Can I have multiple proxy addresses for one user?

Yes, a single user or mailbox can have multiple proxy addresses (aliases) attached to it, but each of those proxy addresses must be unique across the entire Microsoft 365 organization.

Migration & Recovery