To utilize the same alias for Shared Mailbox in multiple domain, Access Exchange Admin center (classic) > Left Pane> Recipients > Shared\Mailbox > click more option to make the alias field available.
Alias is the key parameter of a shared mailbox to make it unique in the backend specially if you have multiple domain.
For instance, I have two domains (domain A and domain B) and want to create a shared mailbox for both respective IT teams. Hence, it@domainA.com and it@domainB.com.
By design, the address name “it” will be unique address in the entire tenancy. Therefore, you need to mask it with an alias for the system to know the two “it” mailbox is unique from their respective domain and so that the creation will push through.
In conclusion, the IT mailbox for my Domain A will have an alias of “itA” and Domain B will have an alias of “itB”.
(Please see screenshot for more information)
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