Non Unique Proxy Address in Exchange Online Error Fixed

Fixing the non-unique proxy address in Exchange Online error can be frustrating, especially if this is your first time encountering this issue. If you dont know, then allow me to tell you. This issue is most commonly seen when you (the admin) try to create or sync a mailbox, but the email address you choose is already in use somewhere else on the system.

Microsoft 365 has a global rule that forces objects to have a unique address. This applies to even those objects that are hidden or orphaned, which are most often also the trigger points of this error. The fixing part is not that difficult; all you need to do is look at the correct place. Let’s start by listing down the reasons why this error appears in the first place.

What Causes a Non-Unique Proxy Address in Exchange Online?

Here are some common scenarios that are known to trigger this sort of error:

Hybrid environment sync issues: This is the most common cause. In organizations where both online and on-premise systems coexist (i.e., AD linked with Entra ID), duplicate entries might get made automatically during user provisioning or even while enabling mail users. The sync will fail, and you will see errors like “The proxy address ‘SMTP:name@domain.com’ is already being used.”

Conflicts within Mail-Enabled objects: It’s not just user accounts that trigger the error. Sometimes objects such as distribution groups, shared mailboxes, or public folders might hold the proxy you are trying to use.

Mail-Enabled public Folders (MEPFs) in Entra ID: If there are any remnants of on-premises public folders that sync to Entra ID but not with Exchange Online, it can trigger hidden conflicts that are only detectable with respective GUIDs.

There are other minor reasons, too, like invalid user provisioning and licence assignment, or legacy infrastructure that has not been properly cleaned up and archived.

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How Does a Non Unique Proxy Address in Exchange Online Look Like?

The message that admins see is of only two simple lines:

“The proxy address ‘SMTP:user@domain.com’ is already being used by the proxy addresses or LegacyExchangeDN. Please choose another proxy address.”

This can pop up as soon as a conflict is detected. Which means during new user creation, while you toggle a manual sync with Active Directory, or even when you restore a soft-deleted mailbox.

Identify Non-Unique Proxy Address

Here is a list of five diffrent methods tested to produce accurate results. Use them to find proxy errors in your own environment.

1. Search Exchange Online Recipient lists:

Connect to the Exchange Online PowerShell and type:

Get-EXORecipient -ResultSize unlimited | Where-Object {$_.EmailAddresses -match "<conflicting SMTP address>"} | fl Name, RecipientType, EmailAddresses

2. Check AD Objects:

Hybrid setup admins can use the following cmdlet directly inside PowerShell:

Get-ADObject -Properties mail, proxyAddresses -Filter {proxyAddresses -eq "smtp:<conflicting address>"}

3. Export All Email Addresses for Analysis:

You can pull a list of all the email accounts and use external tools like Excel to find duplicates. Use the cmdlet:
Get-Recipient -ResultSize unlimited | Select Name -Expand EmailAddresses > C:\all_smtp_addresses.txt

4. Use Microsoft IdFix Tool:

This is a special tool that Microsoft provides you for free. Use it to easily highlight any proxy address conflicts for easy remediation.

5. For MEPFs in Entra ID:
On a server with Entra Connect v2.0

$mailEnabledPublicFolders = Get-ADSyncToolsAadObject -SyncObjectType "PublicFolder" -Credential (Get-Credential)
$conflictingSmtpAddress = "smtp:<address>"
$mailEnabledPublicFolders | Where-Object {$_.ProxyAddresses -icontains $conflictingSmtpAddress} | Select SourceAnchor

This mini script will even identify any hidden public folder conflicts.

Resolve Non-Unique Proxy Address in Exchange Online

Start with the most basic, removing or changing the conflicting address.

Set-Mailbox -Identity <object> -EmailAddresses @{remove="<conflicting address>"}

If you believe that it is public folders that are causing the issue, replace “Set-Mailbox” with Set-MailPublicFolder.

During email provisioning, you could use a temporary external address while enabling. The plan is to enable with a temp address > disable > then re-enable with the desired address in one sync cycle.

MEPF is also a causal factor, so its cleanup is also necessary. Use:

Remove-ADSyncToolsAadObject -SourceAnchor <value> -SyncObjectType "PublicFolder" -Credentials (Get-Credential)

Soft-Deleted or Orphaned Objects: The issue is similar to what happens with old mailboxes in legacy environments, as explained in delete orphaned mailboxes in Exchange 2010.

Conclusion

The non-unique proxy address in Exchange Online error may seem daunting at first, but it usually comes down to a simple conflict with an existing alias. By checking active users, soft-deleted objects, or syncing issues, you can quickly resolve the problem and restore normal operations. And if hybrid complexity keeps causing conflicts, moving fully to the cloud with a dedicated migration tool can save time and headaches in the long run.