Cannot set up shared mailbox - proxy address (domain name) already being used.



I need to add a shared mailbox to a user. A third party set up our Office 365 tenancy and pulled over most of the mail etc. User accounts are synced from our local AD domain to Office 365. We use three domain names. 


The user's account has already been set up. Her default mail account is fine, but I need to add a shared mailbox which uses her name but with a second domain name so that she can manage her mail.


When I try to set up a shared mailbox an error message appears:

add shared mailbox SU HTL annotated.jpg












Her default mail address is and this is specified in her AD account and is shown under Active Users > her account > Username/E-mail. I wish to add a shared mailbox for (redacted in red in the screenshot)


The default domain name for the tenancy is


The error message states is already being used by the proxy address or LegacyExchangeDN of "first.last".


I don't understand it - I have not specified as the domain name for the shared mailbox (it is specified as Can anyone offer any assistance with this, please?


Note that we were not using Exchange or Outlook before we moved over to Office 365.


I'm new to this so all help welcome.


Thank you.





















2 Replies

You probably have this alias assigned to another object already. Office 365 enforces an address policy that requires at least one alias to match the UPN of a user, which might be the culprit here. In any case, just run 




to find the object that has the alias assigned, and adjust it as needed.