I believe the ability to create an XDP email attachment that would fetch the form was removed after Reader 9. It might have been for security reasons.
If you or someone else finds the answer to this one, I would appreciate it if you/they could send it to me.
In designer, we're using a button containing the following xml:
<event activity="click">
<submit textEncoding="UTF-8" xdpContent="pdf datasets xfdf xdp" target="mailto:"/>
</event>
It used to work fine if the form that created the email attachment generated it while the Reader plugin was inside the browser window at the time the email was created.
Is the form at least populating the subject field of the email with the uri? Ours is, using the code above.