Microsoft 365 create a shared mailbox with same alias

When using Microsoft 365 and using multiple custom domains, sometimes you are unable to create a shared mailbox that uses the same alias as an existing mailbox.

The problem of multiple shared mailboxes with same alias

Let’s say, we have a Microsoft 365 tenant with 3 domains;

• domain1.com
• domain2.com
• domain3.com

When you already have a mailbox called “info@domain1.com” you are unable to create a “info@domain2.com” in the portal. The cause of this problem is that every mailbox has a underlying “alias” and that this alias is the same when created in the portal. I have tried this in the Microsoft 365 admin center, Exchange Online admin center and Powershell. I get the following error:

Write-ErrorMessage: ExB10BE9|Microsoft.Exchange.Management.Tasks.WLCDManagedMemberExistsException|The proxy address "SMTP:info@domain1.com" is already being used by the proxy addresses or LegacyExchangeDN. Please choose another proxy address.

The cause of this problem

The cause of the problem is that even if you select another domain in the shared mailbox creation wizard, it wants to create a underlying UPN in your default domain.

How to create those mailboxes?

Luckily i have found out that the solution is very easy and that is to create the new mailbox using the Exchange Online Powershell module. I will explain how this works.

For my tutorial, i stick to the example given above, where i described 3 domains, domain1, domain2 and domain3.

First, ensure that you have installed the Exchange Online Powershell module by running the following command in a elevated Windows Powershell window:

Install-Module ExchangeOnlineManagement

After around 30 seconds, you are ready to login into Exchange Online by using th efollowing command:

Connect-ExchangeOnline

Log in into your account which has sufficient permissions to manage mailboxes.

After logging in, you have to run the following command:

New-Mailbox -Shared -Name "NAME" -DisplayName "DISPLAYNAME" -PrimarySMTPAddress "info@domain.com" -Alias "info_domainname"

Here, we create a new shared mailbox:

• Name: Name of the mailbox
• Displayname: The displayname of the mailbox how it is shown for contacts, users and in the portal
• PrimarySMTPAddress: The primary emailaddress for the mailbox
• Alias: A internal name for the mailbox which has to be unique

You can create all mailboxes like this, and we have to tell Exchange Online exactly how to create the mailbox. After creating the mailbox, it looks like this in Exchange Admin center;

Summary

So creating multiple shared mailboxes with the same alias is not possible in the admin portals which is very stupid. It looks like a way Microsoft wants you to still use their Powershell modules.

I hope Microsoft publishes a new solution for this where we can create those mailboxes in the admin portals and not having to create them using Powershell.

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