Exchange - Duplicate smtp local address in different domain

Scenario

You need to create two shared mailboxes info@contoso.com and info@contoso.uk

The Microsoft 365 Admin and Exchange portals block the second use of info@

Issue

The script underlying the Microsoft 365 portal attempts to create the second mailbox with the same internal alias as the first. i.e. -name "info" -primarySmtpAddress "info@contoso.onmicrosoft.com"

As the alias already exists the mailbox creation is cancelled.

Resolution

Use powershell to override the automatic behaviour

Connect-ExchangeOnline

new-mailbox -shared -name "info-com" -displayName "com-sales" -primarySmtpAddress "info@contoso.com" -alias "info-com" 

new-mailbox -shared -name "info-uk" -displayName "uk-sales" -primarySmtpAddress "info@contoso.uk" -alias "info-uk"

Clean up

When the shared mailbox is created the UPN (username) is set to the default domain name ie: info-uk@contoso.com

To set the appropriate UPN update the Username field in Admin>Active Users properties

The shared mailbox may be assigned an unnecessary SMTP alias. Update the alias list in the Admin>Groups>Shared Mailbox properties.

Note

The commands may produce a slew of replication warnings that can be ignored.