Exchange 2010 EWS view calendar default IPM.Appointment form

Hi, we had a 3rd party add-in that changed everybody's default calendar IPM.Appointment form to IPM.Appointment.MP.  A year or so ago, we ran the EWS script described here:  http://blogs.technet.com/b/heyscriptingguy/archive/2011/12/02/learn-to-use-the-exchange-web-services-with-powershell.aspx to change it back to default.  That worked, however, the form has come back for about 5% of our users.  Rather than run it again against all mailboxes, I would like to identify those mailboxes that have the issue.  So, rather than set these objects, I would like to display the them.  I'm hoping that might help us identify what is different for them and how it came back. I'll paste the code I used from the article to set the property.  I was hoping somebody could give me some guidance as how I could view this property for all mailboxes rather than set it.  Thanks.

Import-Module -Name "C:\Program Files\Microsoft\Exchange\Web Services\1.1\Microsoft.Exchange.WebServices.dll"

$Credentials = New-Object Microsoft.Exchange.WebServices.Data.WebCredentials("username","password","domain")
$exchService = New-Object Microsoft.Exchange.WebServices.Data.ExchangeService
$exchService.Credentials = $Credentials

$mailboxes = Get-Mailbox

foreach ($mailbox in $mailboxes) {
$exchService.AutodiscoverUrl($mailbox.PrimarySmtpAddress)

$Calendar = [Microsoft.Exchange.WebServices.Data.Folder]::Bind($exchservice,[Microsoft.Exchange.WebServices.Data.WellKnownFolderName]::Calendar)

$PR_DEF_POST_MSGCLASS_W = new-object Microsoft.Exchange.WebServices.Data.ExtendedPropertyDefinition(0x36E5,[Microsoft.Exchange.WebServices.Data.MapiPropertyType]::String)

$PR_DEF_POST_DISPLAYNAME_W = new-object Microsoft.Exchange.WebServices.Data.ExtendedPropertyDefinition(0x36E6,[Microsoft.Exchange.WebServices.Data.MapiPropertyType]::String)

$calendar.SetExtendedProperty($PR_DEF_POST_DISPLAYNAME_W,"Appointment" )
$calendar.SetExtendedProperty($PR_DEF_POST_MSGCLASS_W,"IPM.Appointment")
$calendar.Update()