Did you get this error when you were creating a new user? "This email address is already being used as an alias for the shared mailbox <mailbox name>." This article will show you how to remove the email address from the shared mailbox so you can re-use it.
Tip: Email addresses are how the internet knows where to send your email and all email addresses must be unique - across the entire internet no two email addresses can be the same.
Remove the email alias from the shared mailbox
Before you begin, make sure you've noted which shared mailbox you need to remove the email address from.
From the Office 365 admin center, you can find the shared mailbox a couple of different ways:
From the admin center Home page, type the name of the shared mailbox into the search field, and then select it from the list. It will open the shared mailbox's details card.
Go to Groups > Shared mailboxes, and then select the shared mailbox from the list.
In the Name / Email and Aliases section, select Edit.
In the Email aliases section, remove the email address by clicking the remove (wastebin) icon, and then Save.
Note: You can't remove the primary email address. If you need to remove the primary email address, add a second email address for this shared mailbox and click Set as primary. Then you can remove the email alias.
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