I came across an interesting puzzle over at the Spiceworks Office 365 Forums this weekend. A user was trying to add two shared mailboxes to their Office 365 tenant with the same alias.

For example:

  • shared_mailbox@domain1.com
  • shared_mailbox@domain2.com

Trying to create a Shared Mailbox in the normal way resulted in an error.

I tried to do this in my own environment. I already had a shared mailbox called test_shared@cogmotive.onmicrosoft.com, and I wanted to add another one called test_shared@cogmotivereports.com. Here’s what happened.

PS C:\Users\burns_000\Desktop> New-Mailbox -Name "Test Shared Mailbox" -Alias test_shared -Shared -PrimarySMTPAddress test_shared@cogmotivereports.com
Windows Live ID "test_shared@cogmotive.onmicrosoft.com" already exists as a managed Windows Live ID. To use the
existing Windows Live ID, use the UseExistingLiveId parameter.
    + CategoryInfo          : NotSpecified: (0:Int32) [New-Mailbox], WLCDManagedMemberExistsException
    + FullyQualifiedErrorId : A5FDB8CC,Microsoft.Exchange.Management.RecipientTasks.NewMailbox
    + PSComputerName        : pod51016psh.outlook.com

Turns out that there isn’t a very intuitive way to do this. In order to achieve this, you need to create the shared mailbox with a different alias and then change the WindowsLiveID manually once it is created. For example:

Create the shared mailbox with a different alias.

PS C:\Users\burns_000\Desktop> New-Mailbox -Name "Test Shared Mailbox 2" -Alias test_shared2 -Shared -PrimarySMTPAddress test_shared@cogmotivereports.com

Name                      Alias                ServerName       ProhibitSendQuota
----                      -----                ----------       -----------------
Test Shared Mailbox 2     test_shared2         db3prd0610mb356  24.75 GB (26,575,110,144 bytes)

You can now see the mailbox and the UPN it has been assigned.

PS C:\Users\burns_000\Desktop> get-mailbox test_shared2 | select userprincipalname


You can also see this in the Exchange Control Panel.

Shared Mailbox with Incorrect UPN

Shared Mailbox with Incorrect UPN

So how do we set the correct login name? With the Set-Mailbox command like this:

PS C:\Users\burns_000\Desktop> set-mailbox test_shared2 -MicrosoftOnlineServicesID test_shared@cogmotivereports.com

And viola, we have the correct UPN.

PS C:\Users\burns_000\Desktop> get-mailbox test_shared2 | select userprincipalname


We can also verify this in the Exchange Control Panel.

Shared Mailbox with Correct UPN

Shared Mailbox with Correct UPN

And here are our two shared mailboxes, at different domains!

Two shared mailbox with same alias in different domains

Two shared mailbox with same alias in different domains


  • Thanks! Had a different issue, but this resolved it.

  • Thanks! This was exactly what I was looking for.

  • I came across this post while searching for this issue and it worked great! Thanks. But now I’m seeing a different issue and I wonder if you’ve run into it as well. I have my “primary domain” office 365 account that I log into outlook as me@mydomain.com. The shared mailbox I want to open in my Outlook is for a DIFFERENT domain (that O365 knows about, has verified, etc), so let’s say it’s info@otherdomain.com.

    The issue is that I can tell Outlook to “open additional mailbox” info@otherdomain.com (and it works fine), yet when I’m in that mailbox and start a new message in OL, it is from me@mydomain.com instead of the expected info@otherdomain.com. And even if I use the “From” dropdown, info@otherdomain.com is NOT an option to choose there. This does not happen on other shared mailboxes or other external accounts I have attached to Outlook… they all default to the correct from: address based on the mailbox I’m “in” in the tree.

    So while I can send mail to info@otherdomain.com and open/see it within the shared mailbox attached to my Outlook, something is different from other normal shared mailboxes and I cannot make new mail come FROM the shared mailbox.

    Any ideas?

  • Interesting… !
    I have a setup which has the same issue. This workaround will come in handy !

    I noticed some strange side effect – not sure if you have seen this before…

    This is the setup:
    Office 365 main domain: MainDomain.com
    Office 365 additional domain: AdditionalDomain.com.

    Primary SMTP address: referred to with SMTP in upper case.
    Secondary smtp addresses: referred to with smtp in lower case.

    The main domain has a mailbox: user1@maindomain.com (SMTP) and some aliases: firstname.lastname@maindomain.com (smtp) and fn@maindomain.com (smtp).

    When I tried to setup a shared mailbox under the Additional domain, this happened:
    SMTP address: firstname@additionaldomain.com
    Alias (smtp address) auto configured: firstname@maindomain.com.

    I obviously don’t want mail sent to the maindomain.com ending up in the shared mailbox on the additionaldomain.com.

    I noticed, that it is impossible to remove the alias address: not possible using the GUI, but also not possible using PowerShell…

    Any ideas how to get around this ?

    Note that If I preconfigure firstname@maindomain.com on the mailbox on main domain, the setup of the shared mailbox on the additional domain fails (which is kinda the situation you are describing in your article).

    What I think is going to happen, in my scenario, using your approach is that I will end up with again with a smtp address using maindomain.com on the shared mailbox, which is on additionaldomain.com…


    • Just verified: the additional smtp address appeared, but I was able to remove it via the GUI…

  • Thank you, this is exactly what I was looking for. It seems that you can also do this same procedure from the Exchange Admin GUI.

  • I am getting a Warning and an error when I try to do the above. It tells me “uniqueness validation failed” and then does not perform the change. It worked to create the new email but when I try to convert it, that is the error I get.

    Please help

  • Yesterday I renewed into O365 Business Essentials from O365 Small Business. Everything went fine for the mailboxes associated with our primary domain. However some mailboxes on our 2 additional domains are no longer accessible. For example, my primary email is ann@primarydomain.com, and before we renewed I had ann@secondarydomain.com and ann@thirddomain.com. Now just ann@primary is working.

    Each time the O365 support tech tried to set the additional domain mailboxes up as shared mailboxes, we got the following message:

    The proxy address “SMTP:ann@primarydomain.com” is already being used by the proxy address or LegacyExchangeDN. Please choose another proxy address”

    However we confirmed that the referenced proxy address is not used anywhere. He flailed around for awhile, then said he’d get back to me this morning and its been *crickets* since then. So I dug around and found your excellent post, which is describing exactly what I am experiencing! I followed the process described and created a test mailbox in the secondary domain with the same alias as an existing test mailbox in the primary domain, and both test mailboxes now have the same alias, so I believe the process worked with these test accounts.

    However I got this message on PowerShell at the last step, which I do not understand, and which wasn’t mentioned in your post.

    WARNING: UserPrincipalName “testing2@primarydomain.com” should be same as WindowsLiveID “testing@secondarydomain.com”, UserPrincipalName should remain as “testing@secondarydomain.com”.

    And I am worried about trying these steps with my real additional mailboxes, because those mailboxes already exists somewhere and are full of mail (I hope!), and the process described here is for creating new mail boxes.

    Any thoughts greatly appreciated!


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