Exchange 2010 PowerShell: Cannot bind argument - powershell

Question

I'm trying to remove orphaned user objects from all mailboxes in our Exchange server.
When I execute this command:
get-mailboxpermission * | where {$_.User -like "S-1-5-21*"} | foreach {$_.Identity.Name}
It correctly returns a list with all the mailboxes that still have orphaned user account permissions set on them.
However, when I try to remove them by doing this:
get-mailboxpermission * | where {$_.User -like "S-1-5-21*"} | remove-mailboxpermission -identity $_.Identity.Name -user $_.User -accessrights $_.AccessRights -deny:$_.Deny
It returns this error:
Cannot bind argument to parameter 'Identity' because it is null.
+ CategoryInfo : InvalidData: (:) [Remove-MailboxPermission], ParameterBindingValidationException
+ FullyQualifiedErrorId : ParameterArgumentValidationErrorNullNotAllowed,Remove-MailboxPermission
What am I doing wrong?
Thanks for any help.

Answer 1

$_ doesn't work like that, you need to wrap the Remove-MailboxPermission statement in ForEach-Object {}:
Get-MailboxPermission * | Where-Object {$_.User -like "S-1-5-21*"} | ForEach-Object {
Remove-MailboxPermission -Identity $_.Identity.Name -User $_.User -AccessRights $_.AccessRights -Deny:$_.Deny
}
Since Exchange doesn't seem to like nested pipelines very much, you could simply to away with the parameter arguments altogether (Remove-MailboxPermission will automatically bind the permissions from the pipeline):
Get-MailboxPermission * | Where-Object {$_.User -like "S-1-5-21*"} | Remove-MailboxPermission