So recently i had a client with 2 domains. Recently the client, lets call him Fred, asked if he could have a shared mailbox with his main alias on the second domain. I thought this would be easy. So the emails are as follows.

Main Email: fred@example.com
Required Shared Mailbox: fred@example2.com

So i went create a shared mailbox in office 365 but i received the following error.
The proxy address “SMTP:fred@example.com” is already being used by the proxy addresses or LegacyExchangeDN. Please choose another proxy address.

I noticed that even though i had entered the second domain it was giving me an error about the first domain. I figured out it was actually the alias causing the issue.

The was to get around the issue is actually quite easy.


1. Create a shared mailbox with a different alias. In my case i used “fred2@example2.com”.
2. After creating the shared mailbox add the real email as an alias, in my case i add “fred@example2.com”.
3. Then after adding the alias, select “set as primary” on the email you actually want as the main address. In my case “fred@example2.com”.

One thing to remember is that the actual Alias in the Exchange Center of the mailbox will remain fred2 but the email address will be correct.