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: [email protected]
Required Shared Mailbox: [email protected]

So i went create a shared mailbox in office 365 but i received the following error.
The proxy address “SMTP:[email protected]” 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 “[email protected]”.
2. After creating the shared mailbox add the real email as an alias, in my case i add “[email protected]”.
3. Then after adding the alias, select “set as primary” on the email you actually want as the main address. In my case “[email protected]”.

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.