Shared Mailbox Creation Fails

This article deals with a Shared Mailboxes but the work around applies to a standard mailbox also.

The Problem:
When creating a shared mailbox with the same email address for a secondary domain, the process fails.
For example, you have the two domains: firstdomain.com.au and seconddomain.com.au

You have accounts@firstdomain.com.au and want a shared mailbox attached to it to be accounts@seconddomain.com.au
You get the error: The proxy address “SMTP:accounts@firstdomain.com.au” is already being used by the proxy address or LegacyExchangeDN.Please choose another proxy address.

Why the problem occurs:
When you create a shared mailbox – in this case accounts@seconddomain.com.au, it tries to reserve the same alias on the *.onmicrosoft.com domain as accounts@firstdomain.com.au, hence the conflict.

The Fix:
When you create the mailbox you need to also override the default alias address.
In order to do this the mailbox must be created in the Exchange Online Control Center so you have access to all the options.
During the creation select the ‘more info‘ at the bottom which adds the ‘alias‘ box then change the alias to something like accounts.seconddomain or accounts2.seconddomain.
This will then allow the creation of your shared or standard mailbox.

Happy Exchanging 🙂