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Cannot set up shared mailbox - proxy address (domain name) already being used.
Hello 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 ...
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
Get-Recipient user@tenant.onmicrosoft.com
to find the object that has the alias assigned, and adjust it as needed.
VasilMichev Thank you.