find all mailbox delegeations for user
PS C:\> Get-Mailbox | Get-MailboxPermission -User vasil
Identity User AccessRights
-------- ---- ------------
HuKu Vasil Michev {FullAccess}
retail Vasil Michev {FullAccess}
sharednew Vasil Michev {FullAccess}
testplan2 Vasil Michev {FullAccess}
WC Vasil Michev {FullAccess}