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Get-MailboxIRMAccess

Use the Get-MailboxIRMAccess cmdlet to view delegate access to IRM-protected messages in other mailboxes (shared mailboxes or user mailboxes where delegates have Full Access permission). Exchange PowerShell reference

From the Exchange PowerShell reference 2 parameters Its page in the reference

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Parameters

2 in the Exchange PowerShell reference
-IdentityThe Identity parameter specifies the mailbox where you want to view delegate access to IRM-protected messages.
-UserThe User parameter filters the results in the mailbox by the specified delegate.