How to fix conflicting proxy addresses in O365 when creating a mailbox

Error: The proxy address "SMTP:name@domain.com" is already being used by the proxy addresses or LegacyExchangeDN. Please choose another proxy address.

The proxy address is already being used by the proxy addresses of another mailbox

Why can’t I create a mailbox at [email protected] if one already exists at [email protected]

The Problem

So, you have two (or more) domains associated with your Office 365 Exchange account. When you try to create a shared mailbox on one domain, you encounter an error stating the proxy address is already being used.

For example

You have two domains, lets call them “domain1.com” and “domain2.com

You’re trying to create a shared mailbox on domain2.com and you’re hitting an error.

The message reads:

The proxy address “SMTP:[email protected]” is already being used by the proxy addresses or LegacyExchangeDN. Please choose another proxy address.”

The proxy address "SMTP:name@domain.com" is already being used by the proxy addresses or LegacyExchangeDN. Please choose another proxy address.

The Cause

This issue occurs because the Shared Mailbox we’re trying to create is automatically assigned a UserPrincipalName on the default domain for the organisation, no matter which domain we try to create it on.

For example.

You have a user, shared or resource mailbox already in existence on domain1.com with an alias of [email protected]

When you try to create the [email protected] mailbox, Office365 tries to assign the UserPrincipalName as [email protected], this is already in use so O365 throws the error that we’re seeing.

The Fix

1: Connect to Office365 / Exchange as an admin via Powershell

2: Remove the alias from the existing mailbox (don’t worry, we’ll add it back shortly!)

NOTE: If the conflicting alias is the default SMTP for the mailbox, you’ll need to create a new temporary default SMTP alias before removing the conflicting one

3: Create the shared mailbox you want to create on domain2.com

The value for “MicrosoftOnlineServicesID” is the value of the MailBox Property “WindowsLiveID” and this defaults to your default domain, as explained in The Cause – https://msdn.microsoft.com/en-us/library/ee423637(v=exchsrvcs.149).aspx
We want to change this to our secondary domain to enable us to delete the default primary domain smtp alias.

Ignore the warning that appears after you run the command.

4: Remove the default smtp address from our new mailbox

5: Confirm that the removal was successful

6: Add the domain1.com alias back to the email address.

9 thoughts on “How to fix conflicting proxy addresses in O365 when creating a mailbox”

    1. Aiden Logi Arnkels-Webb

      This issue with 1 domain strikes me as odd. Could it be conflicting with the .onmicrosoft.com domain?
      Have you completed Domain setup in the admin centre?

  1. Step 3 is very confusing, maybe mention to create the shared mailbox from the admin portal or something, from these instructions I assumed that step would be included via Powershell.
    And your code in step 3 should say [email protected] instead of [email protected] as it was confusing as to which email address to use there.
    Step 5 should be @domain2, not @domain1 as it hasn’t been added back yet.
    Otherwise thanks for the info.

    1. Aiden Logi Arnkels-Webb

      Sam, good spot and thanks for commenting – I’ve updated the guide and run through instructions successfully.
      Does that make more sense now?

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