Exchange - Shared mailbox on secondary domain

Ref https://www.quadrotech-it.com/blog/create-shared-mailboxes-with-same-alias-at-different-domains-in-office-365/

Issue

Attempting to create a shared mailbox with a similar address to a Universal Principal Name raises a duplicate alias/proxy address error.

PS> New-Mailbox -Shared -Name "Anne Other" -Alias Anne_Other -PrimarySMTPAddress anne.other@contosocorp.com
Windows Live ID "anne.other@contoso.onmicrosoft.com" already exists as a managed Windows Live ID. To use the UseExistingLiveId parameter.

Workaround

To suppress the error and allow the shared mailbox to be created

1. Create a shared mailbox with a unique alias

PS> New-MailBox -Shared -Name "Anne Other corp" -Alias Anne_Other_Corp -PrimarySMTPAddress anne.other@contosocorp.com

2. Change the WindowsLiveID manually

PS> Set-Mailbox anne_other_corp -MicrosoftOnlineServicesID anne.other@contsocorp.com

Scenario

Microsoft 365 / Exchange users may require a similar email address on a secondary domain

Eg.  Anne Other works for Contoso and has the primary email address and user principle name (user name)

anne.other@contoso.com

The initial Microsoft domain was contoso.onmicrosoft.com, hence Exchange assigns an implicit alias

anne.other@contoso.onmicrosoft.com

The Contoso company launches a new identity, Contoso Corporation with the domain name contosocorp.com.  Microsoft assume that Anne will be fine using an explicit alias.

anne.other@contosocorp.com

However, the explicit alias causes messages to the new domain to be delivered to Anne's original anne.other@contoso.com inbox.  This confuses Anne and does not suit the workflow.  Anne would prefer to use a shared mailbox dedicated to the new business identity.