Shared mailbox with the same email (local part) can't be created to a different domain

Occasional Contributor

Hello everyone,


I have a shared mailbox


The problem: When I'm trying to create a shared mailbox this results in an error saying that each mailbox should have unique name and that this name is already in use.

I have verified both domains an set up dns correctly. Doesn't O365 separate email addresses from different domains even if they have the same local part (support)?

Is it even possible to have the same local part for emails from two domains?

I need an email address for support for all of my domains and since I'm migrating from another service, they all must be in the form

Thank you in advance for your input, Matti

13 Replies
Seems like a similar problem. The linked post is quite old though.

I have no idea how to replicate the solution in office 365 admin center.

Simply use PowerShell, the UI is limited for this scenario.

Without Powershell experience in a mac environment with instructions from a 5 year old blog post doesn't seem like a great combo.

This scenario seems so obvious that there must be an easier way?

Its two domains having an email address for support. Shouldn't be that complicated.
I will do some testing when I get home from work


ok! i've managed to create 3 shared mailboxes with the addresses of , and My setup was exchange online only and i did it from the exchange admin center only.


Go to exchange online admin center -> recipients -> shared : click the plus sign

Here you can type the name and also alias if you click the  "more options" button..(see attached file)

Type a name and alias different to the mailaddress, like support_uk or  something and set the mailaddress to ex.!

Repeat this when creating another shared mailbox:

name/alias: support_us



tested and confirmed working in my test environment





I found the option to type the alias. But I get this error:

The proxy address "SMTP:asiakaspalvelu@domain2" is already being used by the proxy addresses or LegacyExchangeDN of "Domain 1 display name". Please choose another proxy address.( The "UserMailbox" with display name "Domain 1 display name" is already setup and using the same address.)

Are both the name and alias of the domain2-mailbox you try to create unique?

best response confirmed by Matti Pekkarinen (Occasional Contributor)

Moi Matti,


The problem is that when you create a shared mailbox, the alias with the local name is also created. Thus, regardless of the domain part, you have the error you described.


Luckily the workaround is quite simple. Just create a shared mailbox with unique local name, such as After it is created, edit the mailbox email addresses to change the default address to and you're done!

Another work around is to create a user mailbox from azure Active directory and assign a license

Then convert it to a shared mailbox and remove the license

@Matti Pekkarinen 


Hi Matt,


So I think i found out how to do it but its a bit of a workaround. Though this may be wayyyy too late but in case someone else out there is like me...

In summary : You can create a normal user account and then convert it to a shared mailbox.


Note that the spaces between addresses are actually full stops '.'


I have User: John with User account:

john @ domain1 com already in the system

I want a shared mailbox called John @ domain2 com

I also have domain1 .onmicrosoft com (which have had from the start)


To create a user with the same alias but a different domain I needed at least 3 different domains to choose from in the drop down as 365 seems to create an alias with the same name in the previously selected drop down domain. So I go to create a user account for Johns shared mailbox (aka second domain account). The drop down however is already populated with @Domain1 com. So I enter his name and I change the dropdown to Domain1 . onmicrosoft com. Its like Nahhh cause there already is an alias in the system with @domain1 com. Thats fine now I change to @Domain2 com and as there is no account in the system with John @ Domain1 onmicrosoft com so it lets me do it. Once the account is created is creates an alias with an @onmicrosoft com address for that account (this can be deleted if you want)


I hope this makes sense and works for someone else too.


Have a great day