$usertosearch = "max@muster.de"
$sharedMailboxes = Get-Mailbox -RecipientTypeDetails SharedMailbox

foreach ($box in $sharedMailboxes) 
{ 
    $perms = Get-MailboxPermission -Identity $box.Alias

    foreach ($perm in $perms)
    { 
        if(
            $perm.User -like $usertosearch) 
            { 
                write-host $box.PrimarySmtpAddress 
            } 
    }
}