A cloud-based service included in Microsoft 365, delivering scalable messaging and collaboration features with simplified management and automatic updates.
There's no command could make sharedX existing in Admin1's mailbox. It's by-design phenomenon of mailbox permission.
"And according to the rule sharedX mailbox should automap under the sharedY folder in the outlook of user Admin1's mailbox."
-Actually, Full access means Allows the delegate to open the mailbox, and view, add and remove the contents of the mailbox. It doesn't mean Admin1 can open/view those mailboxes that delegated by SharedY. If SharedX exsits in Admin1's mailbox as you expected, you are full accessing a mailbox without permission, Microsoft doesn't want to see it happening.
According to my test, no matter the shared mailbox is mapped due to automapping or manually adding it in "Account setting", you can't view the mailboxes delegated by SMB2:
By the way, I've also tested replacing SharedY with a user mailbox, same phenomenon as above.
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