Sending read receipt via changing the IsRead property of an EmailMessage

Hi,

I set the IsRead flag of a number of EmailMessage objects.

After that I call the UpdateItems method to update the specific items on exchange side.

Some seconds later the changed items also change within the outlook client.

But no read receipt will be send out if one of the emails have the request for.

Is there anything to do additionally to send out read receipts only via setting the IsRead flag to true?

Thanks for your help.