EWS error updating item.

Hi, im using the following code to remove the read receipt from a received email that is in the inbox.

$item.IsReadReceiptRequested = $false
$Item.Update([Microsoft.Exchange.WebServices.Data.ConflictResolutionMode]::AlwaysOverwrite)

However i'm getting the following error below.......can you advise where i've gone wrong please?

Thanks

Exception calling "Update" with "1" argument(s): "Update operation is invalid for property
of a sent message."
At line:217 char:7
+                         
$Item.Update([Microsoft.Exchange.WebServices.Data.ConflictResolutionMode]: ...
+    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
    + FullyQualifiedErrorId : ServiceResponseException