Forget mailbox name in Outlook which command let me knows on which mailboxes i permission have. Exchange 2010 server

I need a command for powershell thats give me a list of mailboxes where i permission for have.
I work at a company with 300 users the problem is that one of the users forget the name of a mailbox he want to add.
So i need a command that ables to show me an list whit his mailboxes.

Whe use exchange 2010.