Hi
I know this is an old thread but I had a similar situation and was able to remedy this by creating a transport rule in Exchange:
Go to Exchange Management Console, Organization Configuration, Hub Transport, Transport Rules
Create new transport rule, give it a name, Read Receipt Rule, Select "From users that are inside or outside the organization, or partners"
Change "Inside the Organization" to "Outside the Organization"
Select "Sent to People", click on "people and add the queue mailboxes affected
Click Next, select "Remove Header", click "header" and type "Disposition-Notification-To" without the quotes
Click Next, Next, New.
You will no longer have the issue.
Thanks
John McGregor